Best plugin to manage SaaS subscriptions

This may be a better question for the forum, but in hopes of getting a faster response, what is the best WPMU DEV plugin to use for this situation:

We have a web application (not in WordPress), but do not yet have online billing for customers. The business model is subscription-based. We want to provide an online portal via WordPress where customers can signup for accounts, make a payment, and see billing history. Affiliate links and coupons would be nice too.

What is the best plugin for this case -- membership or pro sites? Or ecommerce?

Thanks!
-Allen