There is no choice!
XP home or Pro.
(Waits for vista fan girls)
Newegg or zipzoomfly. Zipzoomfly might have free shipping on it, which "may" make it cheaper.
Now to be "slightly" more serious.
It really depends what you want to do.
If you want to play alot of new games and we make the assumption that DX10 will on be for vista and we make another assumption that game creators will not release a DX9 version of the game. Then the Choice is clear. Vista.
If we make the assumption that DX10 will eventually come out for XP OR game creators will release DX9 versions of the game as well. Then XP.
Id buy XP now, and Wait till you absolutely, 100% needed vista to run new games or software.
Or you could, cough, um, always cough, download cough both, or either or... Cough.
So really, the question is, will you be Playing new Games and is DX10 a must for you? If yes, and you don't want to wait for DX10 to show up on xp (Reguardless of who actually develops or releases DX10 for XP) then Vista.
As much as it completly disturbs me to say it..