Which is Better, 32-bit or 64-bit?

There is no upgrade path from 32 bit to 64 bit so if you want to install 64 bit, it will have to be a clean install.  Personally I wouldn't go back to a 32 bit system on a 64 bit PC, and almost all my PCs run Win7 64 bit except my netbook which only has a 32 bit CPU (Atom N270) so it can't, and it's also only got 1gb of RAM, but the other notebooks and desktops all run 64 bit operating systems, whether they run Windows 7, Windows Vista or Server 2008 R2.  They all have 4gb or more (and in most cases more) RAM.  One has 12gb, one has 8gb and one has 6gb.  I do a lot of video editing and encoding. and the extra RAM helps, but if you don't need 64 bit and your 32 bit installation is stable, it's probably something to think about staying with 32 bit until you really need it, just for the avoidance of the format of the primary hard drive and reinstallation of every piece of software you have on your PC.

It comes down to what you do with the computer and what hardware you use with it. Without having added any memory, 64bit made the tiniest increases in some aspects of the WEI scores (+0.1 for RAM, CPU) over the 32bit OS. Not to say that there aren't other advantages which might justify the cost of purchasing the 64bit version but there could be other downsides in terms of owning existing hardware that might not be supported in 64bits and existing software that might not gain any advantage from it.