Stripe will probably beat PayPal if it finds a way to expand internationally quickly.
Stripe seems to be the perfect one, wepay looks like a great competitor to PayPal, in fact all the great payment processing companies are US only, I'm desperate finding a good solution available in europe, working in France as far as I'm concerned.
Finaly, I'll go with PayPal, just because it's the only one working worlwide without a merchant account.
I'm using Skrill Moneybookers in adjunct with PayPal, but not to replace it (yet).
I use Payoneer.com to pay my Russian developer. He seems to like it.