The SANS Institute has published an interesting paper about Vista vs XP SP2 security features for laptop users.
The paper compares existing (and improved) features and the new features added in vista (like Bitlocker or UAC).
It is interesting to read as a fast overview, and as it concludes, almost every user should choose Vista instead of XP if buying a new laptop, because you get for free a lot of security enhancements.
I'm with that opinion. Unless you're going to play with the laptop, or you have an old one (single core, less than 2 GB RAM and bad graphics card), choosing Vista is correct, but upgrading from an old laptop can degrade the performance noticeably (as happens with my work laptop, which suffers the lack of horsepower a lot).
Now that I've got my new laptop for development, with Dual Core, 2 GB RAM, a laptop SATA HD and a decent NVidia graphics card, Vista goes very very smoothly even when doing intense use (moving GB of data, installing software and other CPU/HD intensive tasks). RAM consumption is still a bit high (800-900 MB with just Firefox and 2 IM clients), but the fast HD helps a lot to speed-up the memory paging.