RTR 6.1 has been like a fresh game to me, so I'll stick with it until BI is out and patched. I'm not worried that BI will not be a good game, but I'm sure the old same 1.0 bugs will need to be addressed, and I'm in no hurry. The dual install idea also sounds like a good idea, and I've never had registry problems with other dual installed games. The price is another good issue. BI seems to have a full game price for an expansion game, but maybe that's the way things are heading and CA is just getting out there first.

BTW I'm definitely waiting on Civ4. It seems very ambitious, and that often translates into the need for several patches. I really want Civ4 to be great, but they kind of annoyed me with Civ3.