D&D High-Level Campaign, which edition should I choose?
4th Edition. There's a smaller community, yes, but part of why 3.5's community is so large is that the rules are much more open (which I know is considered a feature), which results in a wide range of preferences and approaches, on top of the lack of built-in balance. So, it's hard to imagine a 3.5 game having any kind of coherence - as you say, you'd have to just roll with the crazy. I wouldn't even bother with actual rules, just assume the characters and their opposition are capable of pretty much anything and have fun describing that.
That said, I don't have much experience with actual gameplay at epic levels in 4th edition, though I'd like to have some. I gather that the game becomes very easy for the PCs if the challenges are kept the same as at early levels, i.e. everyone on foot and in range and nothing crazy about the environment. My understanding is that the PC options and combos synergize like crazy, whereas the monsters don't have as much of that, so they get steamrolled.
But I don't know that firsthand. Keep me posted and let me know if you would like help making this happen.