Maemo is an impressive OS, with the caveats being that it's more of a mini-computer experience than a phone experience, and Maemo dies with the N900 since Nokia is moving to Meego.
Symbian ^3 is more phone-like and more Nokia-like. It's also here to stay for a while. In that way, the E7 is a more solid bet for the future than the N900, and ^3 is likely to get updates from Nokia.
Meego looks very intriguing, though again it's a bit more on the mini-computer end of things in terms of UI right now. It's hard to make a firm statement about it because the final OS isn't out yet and we won't see Meego phones until 2011.