Shannon, thanks so much for your reply. Quite wonderful that there is actually something to choose from!
What I find slighty tricky is the co-dependence between the various parties. If at one point, one of the product providers decides to call it a day and stop providing version updates, then how much hassle would you looking at? For instance: are membership paid subscription statuses tracked in the membership module or at the payment service provider?
For that reason alone –as paid subscription membership is a feature that not only involves security but also absolutely requires some sort of guarantee in continuity– I feel that the best route would be a First Party (EllisLabs) module, or a certified Third Party module.
If EllisLabs is indeed not actively planning or working on such a feature, at the very least it would be a good thing if EllisLabs –and this may already be the case– provided developers with an API that at least left the subscription status on EE’s membership side, making switching between these various payment systems a smooth ride.