Depends on what you need with International. WPP works with several currencies and you pay whatever Paypal's going exchange rate is that day (on top of the transaction fees). For instance, we take orders in Canada. I request USD and get paid in USD, even though they pay in CAD. WPP is a little restrictive in that you can only take money where Paypal accepts it.
The big difference is going to be that PayFlow is a gateway only, while WPP is a merchant account and gateway in one. A third party merchant account might have more options, if more currencies is more important to you.