I'd say there's a significant performance difference between 2.5 and 2.2, especially if the 2.2 is auto, which really takes the edge off the 4 pot. There is another significant difference going up to a 3.0 from the 2.5. If you're going auto, then the bigger engine will always be better.
All the V6s are pretty much of a muchness in terms of reliability, etc. IMHO As said, there was a phase around 1998/9 where 3.0s tend to spring coolant leaks from the rear of the head gasket, and around 2001 there was a batch of dodgy valve stem oil seals on the 2.6. These are minor issues that have affected relatively few cars, in my opinion. Get one that has been looked after and you should have few issues up to 200K and beyond. There are differences in the ancillaries between 2.5/3.0 and 2.6/3.2 but overall, some changes for the better, some for the worse - probably not a lot in it.
As engines go, the V6s are pretty solid, by my reckoning. They get a bad reputation for access to some parts being awkward but once you know your way round one they are fine.