Windows 7 is simply a much better OS than Vista and going 64 bit offers more memory and some speed boosts over 32 bit.
Keep in mind that you can only do a regular upgrade bit for bit. As in Vista 32 bit to Win 7 32 bit or Vista 64 bit to Win 7 64 bit. If you are going from Vista 32 bit to Windows 64 bit, you'll need to do a fresh install.