There is no significant upside to moving to a (monthly) allowance.  To be fair, there may be some small conveniences to you:
1. You can set up recurring auto-pay from your bank to hers (or whatever app to whatever app)
2. You won't need to grab cash at an ATM before you see her
3. Can reduce the awkwardness of handing her cash and feeling a bit "hooker-ish and John-ish"

 
But IMO these are only slight positives compared to possible down sides:
1. Volume discount? Nope. She won't accept it, and when you actually try to implement it, you will find you are paying MORE when you do the math because,
2. Monthly is premised on the idea of a fixed, regular schedule like 2 times a week or 8-10 times a month. But life happens and you will have difficultly always getting your 8-10 sessions a month. You may need to travel, you may have family visiting, work can require additional time, she gets her monthly cycle (which takes her out for 5-8 days per month), she could travel, etc., etc., etc. So,  
3. If you need to miss a date, you have already paid and there will be no refund.  
4. If she missis a date, you have already paid and there will be no refund.  
5. Your risk of loss increases from the PPM cash to the full month's cash. There is nothing stopping her from taking the full next payment and ghosting you forever. And before you say "Not her!", I say "Yes, her!" It's happened to me, more than once.  
6. Her incentive to deliver mind- and dick-blowing experiences to you is gone.  She gets her money no matter how poorly she delivers. And this will happen; or she will not be as available for longer dates, or she won't be wanting to do all those freaky/kinky things you like because (excuse). She will slide towards doing the minimum needed to get you off and she will be out the door before you can towel off and grab your tighty-whities.  
7. She will slowly start to believe that she is entitled to that monthly payment forever, and if you ask her for more time, or to do something new she will want additional allowance for that "special" or "incremental" activity (because it's: more time, she has to cancel something else, she feels extra effort deserves extra cash, etc.).  
8. She will gain weight, stop dressing up for you, start being late and leaving early.  

 
I could go on....  

 
Ok, ok. I sound like a bitter, pessimistic asshat, right? Except: These are all real-life consequences I have experienced for moving from PPM to Monthly.  

 
I will close by reminding you of the NUMBER ONE RULE: NEVER SEND CASH IN ADVANCE.  A monthly allowance is exactly that.   Paying for 4, 8, 10+ BCD's before they happen.    

 
Ok, so you say "Hey, I'll just give her the cash at the end of the month!"  But logistically, that only really applies in the 1st few months.  Once you've gone through a few monthly cycles, its functionally the same as paying 1st.  

 
Life is good

 
The Cat

I agree with everything Herb has said, but here's some more thoughts about feelings of obligation/entitlement, both of which are serious buzz killers in an arrangement.  Here's a quote from Jessica Fern's excellent book "Polysecure": "I find security in the fact that when I'm in relationships I know that my partners are not with me because they are obliged to be, but because they continue to choose to be."  She wasn't talking specifically about Sugar relationships, but she might as well have been.  When you do a ppm arrangement, you both know that when you show up for visits you both actually want to be there, not because you are obliged to (cuz you already got paid)  or because you are entitled to (cuz you already paid).  PPM feels way better and in my experience leads to a deeper, genuine, longer lasting connection.

Allowance is a disincentive to her for all the reasons mentioned above.
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But you really only have to look at your own appetite and availability. There's no way I could make an allowance benefit me schedule wise. Now add a rotation to that.  
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Unless you are sufficiently wealthy that money means nothing to you, ppm is going to be more economical in the long run.

Allowance would take your relationship up a notch. The only advantage would be if you both really want to spend a lot of time together without counting.  
Is she going to spend the weekends with you? Leave her clothes at your place. Do you plan to be exclusive and test?
If not, see the advice above.  
The other question is how to handle this gracefully. I’d suggest saying you enjoy things now but your relationship is not at that stage .. yet. Yes it kicks the can down the road but you can keep doing this a while until you figure out if you’re there.

Although I'm pretty new to sugaring, from everything I've read and seen, PPM is by far the better choice. It's just too easy for an SB to get lazy and not deliver anything short of amazing. You'll hear all sorts of SB's talking about 'allowance' on online forums and such and to me it's nonsense and a prescription to get rinsed. Some say PPM is 'too transactional' but to me the whole sugaring dynamic is, at it's core, a transaction! [No matter how you want to 'sugarcoat' it, he he he.]  

I realized that I failed to add a useful strategy when an SB (or a POT) asks for a monthly allowance.  I stick by my opinion that PPM's is the best and only way to go. But how to get her onboard when she pushes for monthly?  

 
Here's a technique I've used successfully in the past:  

 
First, take her request seriously.  It's important that you "validate" her opinion and needs, even if you know you it's not going to happen.  Next, ask her to talk it through with you so you can understand why she wants it. Do you really care why? Fuck no! But her explanation will give you the relevant talking points you need to turn this in the direction you want.  

 
Now that you have acknowledged her "needs" and made her feel validated, it's time to take her through the process:  
0. If you haven't already done so, explain that as a caring and nurturing SD, you will always prioritize her "needs" first, and her "wants" last. What's the difference between a "need" and a "want?"
- She wants a new $80k BMW, but she needs a $15k serviceable car
- She wants a $4k Dior bag, but she needs a $45 bag from Target
- She wants a 7-day cruise to Acapulco, but she needs to cover her rent.  
- And by example when discussing, you want a Honda Enduro 500, and you got one by prioritizing your needs over your wants until you could get that bike.  
Comment that you will always strive (great word because it's "aspirational" and not a fixed commitment) to help her with her needs. Next,  

 
1. Ask her to help you break down her "needs":  
- Rent, car payment, insurance, phone, etc.  

 
2. Total it up and ask her "if you could have these covered or nearly covered every month, would that help you?" There is only one possible answer: Yes, Daddy.  

 
3. Now you have a monthly target "allowance" you can break it down into segments that equal your monthly schedule.  
- Let's say the target is $2000/month.  
- She may tell you, or you can assume that she is already able to generate all or most of that by herself (perhaps with help from parents or another SD).  
- You can offer to cover "as much as "xx% to yy%" of that as her SD. So ask something like this:  
"If we could meet once a week and I could help you with $400 each meeting, that would cover over 75% of your needs, right?" She will agree because - math.  
- Now point out some weekly will mean as many as 5 meets in some months. "Are you ok with that?" (this is an important question!)
Based on her response, you can then start the "what if" questions:  
- If that happens, are you ok with the same allowance?  
- What if you she needs to miss a week for some event (family, work, project)?  
- What if I need to miss a week?  
Eventually she may back off her ask, not wanting to miss the upside of that 5th date, or she may be ok with it as is. That when you lock it down:  

 
4. Thank her for "working with you on planning" blah, blah, blah.. Then suggest a "trail run" where you target your agreed to monthly amount, but you want to start off with installments - just to make sure each of you are treated fairly,. Yes, it's all just slight-of-hand agreeing on what you are already doing, but she got to solve this question together with you.  

 
Life is good

 
The Cat