We usually do Epcot if it's going to rain. Sure, Test Track won't be open, but once you're inside Future World attractions, you're in them for a long time.
As for DisneyQuest in the rain - we've never done it, because we're always afraid it's gonna be packed. From what I've read on these boards, that seems to be the case, but like I said, we've never done DQ when it's raining.
RyMickey