On the other hand, because you have XP on your mini, you can buy the Upgrade of Win7 for a more reasonable amount.
My mini dual boots OSX 10.6 and Win 7 (RC - the pre-release I had lying around), and they're both brilliant. I installed the bootcamp drivers from the Snow Leopard disk on Win7 and it can happily view the MacOS partition, and vice versa from OSX.
Personally I find that Win7 boots faster, though OSX is my primary OS on my mini. OSX recovers from sleep faster, and doesn't prompt me to log in every time it wakes or starts up, but Win7 does (though it's a configuration thing I haven't changed yet).
OSX prompts for my password more often when apps want to make changes, which is synonymous with Win7's User Access Control, though that is just a Yes/No dialog, so if you have a complicated password like I do, it's a bit nicer. As an old hand to OSX I'm sure you're used to it anyway so it's likely not a big deal.
Both Operating Systems seem similar in terms of reponsiveness once they're running.
All in all, very pleased
(though I can't figure out why my bios settings keep resetting... very annoying)