I'm uncertain about how PayPal's fees work in regards to refunds/ chargeback, so somebody else can probably clarify that; however, my personal opinion as a client would be that both choices are pretty restrictive and a bit off-putting. There are instances in which something pops up in which the client is no longer able to pay for the commission. Maybe it's an emergency, or maybe they're just no longer interested for whatever reason, but the idea of not being refunded in either case doesn't sound reassuring.
If, for example, you're partway through the commission, but an emergency pops up in which the client needs the money that they initially had to spare for a commission but is now unable to ask for a refund. It can be frustrating and even stressful for both parties involved. What I would suggest is having certain clauses instead. For example, a certain percentage is refunded depending on the amount of work that had already been done. That way, the client is still able to get some of their money back, but you're still able to keep the amount for the time and work you had already put in the commission.
That's my two cents at least, but as the one providing the service, it's up to you how to set up your TOS and for potential clients to choose to agree with it and commission you or otherwise pass it up. I hope you're able to decide on something that you're satisfied with, and best of luck to you!