On my system, Vista hits my HDD on start up for about 3 to 4 min.. after that, it hardly accesses my HDD unless I am running the TV Tuner (MCE works as TIVO so records current episode you are watching, then deletes it when tuner is turned off), playing games such as Crysis, Fallout 3, Hellgate London....
Other than that, I experience less HDD access than XP does..
However speed between Vista and XP.. I have to give them the same rating
Visual between Vista and XP.. if you want true HD and effects from movies, games, etc.. Vista hands down.
Compatibility.. this I have to give to XP.. however I have only 10 titles that will not work on Vista out of 165 games and 86 other software programs.
Not Vista Compatible : SFC II titles Dynaverse, Dominion Wars, Star Trek : Hidden Evil, MS Visual C++ 6.0 (You can install it and program in it, but programs do not compile correctly), MS VB 6, 3D Studio Max 3 and 4 and 5, Artifex (SFC map editor), Diskeeper (older than 2008 version).
Some programs need some tweaking in Vista in order to get full functionality.
Vista resources.. with all graphics enabled, AERO, enhancements, animated background (Vista DreamScene) .. memory being used.. 1.48 GB at 3% processor usage (Intel Core 2 2.66 Ghz w/ 1333 Mhz FSB OC'd to 3.86 Ghz w/ 1975Mhz FSB)
a few more resources than XP requires, however the trade off is a big jump in graphics (not just eye candy of the OS, but DirectX 10 shader effects in games as well as true HD playback from TV Tuners [if tuner is HD compliant]).
I wouldn't recommend Vista for anyone running less than 2 GB on their main board.. or if you want fast usability, 4 GB system memory and 4 GB USB memory stick set to READY BOOST.
Also many people turn off SuperFetch.. I have mine enabled, which is why I hav somewhat prolonged start up reads, but once the system caches to the Ready Boost.. nice and fast. and physical memory is then reduced to 1.03 GB in use.
I can get that down to about 800 MB by disabeling AERO and most of the shadow effects (reducing the system down to XP levels) ..
Vista Ultimate is the only version of Vista to have if you plan on running Vista.. I have had less compatibility issues with programs under Vista Ultimate than I did under Vista Home Premium. I didn't have all that many problems under Home Premium.. but I have found Ultimate to be more user friendly.