I dual boot XP Home SP2 and Vista Home Premium x64. Comparing a mature install of XP (no bugs, stable, very well maintained) and a fairly mature Vista install (both running the same programs on startup), Vista boots faster. Whatever the official facts are, I can definitely count how long it takes to boot.
Also bear in mind here's a wealth of difference between Vista x86 and x64. I use x64 on mine and really have a hard time faulting it. Driver support is pretty good now - Creative still needs to get their finger out, but then their XP driver is mediocre anyway. Interface is fantastic, makes XP look something Fisher Price dreamt up...
I've not tried Crysis in XP yet (like someone said above, you'd have to force Vista to run it in DX9 mode at exactly the same settings to get a fair comparison), but most other games either run at the same speed or marginally faster in Vista for me. FEAR at identical settings is 3fps faster (average, with Alchemy enabled), and about 10fps faster on the max, NWN2 gained about 5fps, and TR Anniversary is also smoother. I can't give a 3dmark 06 comparison as XP defaults to a lower res than Vista (no, I'm not buying a pro license).
Part of the problem with Vista is people trying to run it on under spec hardware. We bought a laptop with 512mb ddr and a celery m (not my recommendation!). There were tears. Whacked XP on and all rosy.
I do agree on the service pack though - Vista SP1 is little more than a collection of Windows updates, whereas XP SP3 as another evolution of the OS. I can't see why M$ would do something this stupid (commercially), as they could have delayed XP SP3, and dumped the resources into Vista SP1...