Might be too late now, but I think people around my office would recommend iFix hands down. We use it for a lot of our systems. For me, I think Wonderware is great. It's easy and quick to develop. The hierarchy of derived templates is something that iFix is only just now catching on to. However, Wonderware has a number of quarks and since it is more user-friendly, that means it can be hard to debug. iFix is much more open in its format. I hear that once you get all the quarks worked on in Wonderware, its solid and requires little maintenance, but I haven't gotten to that stage yet. The transparency of iFix means it's easier to see what is happening and why. It uses VBA and pretty standard communication while Wonderware uses its only brand on network protocols. It sounds like you are looking for reliability above all else, so that's why I said iFix. Wonderware just had an overhaul of their system (~2010) so there might still be some bugs. Hope I helped.
