The speed differences between XP, Vista, and Win7 are trivial in my way of thinking. To me "faster" is 2x, not 10%. Processor and Mobo choices would affect speed a lot more than OS choice... Win7's UI is on a par with a MAC. I'm going to wait until it seems to be stable and get all new machines with Win7. Currently I'm running XP on an older machine that wouldn't quite stream HULU videos with Vista and Vista on the trading machine... I don't have any big complaints about either OS really but the UI is really important to me and improved file management can't be a bad thing...
Choosing different levels of UAC is a big thing really, for the surfing machine, which I'm going to rebuild from a hdd format up periodically anyhow, I can set it to a low level of security. For the trading machine which I need to guard with a level of paranoia that makes sweat bead on my forehead, it can be set to the other extreme...
I'm feeling good about a Microsoft OS for the first time since XP came out...