I know other will not agree with me, but.....
Your are more assured of backwards compatibility (with older software) with Win7x32. Not only app software but also drivers. 32 bit drivers will not work with 64 bit OS. So if you have any peripherals that you don't want to replace (e.g. scanner, printer) that are quite old and only have 32 bit drivers you will need to use 32 bit.
The main advantage of Win7x64 is that it can support more memory (more than you will ever need). Whereas Win7x32 will only allow access to approximately 3.5GB regardless of how much you have installed. If you do not need to run memory hungry apps (such as video editing and "big" games), then 32 bit may suffice.
I have Win7x32 installed on a machine with 8GB and I get 3.488GB usable - BUT, I never get anywhere near using even that 3.488GB, so x64 would not do anything for me.
Other advantages of x32:
- Uses less disk space (and disk transfers are faster as less to move around)
- Windows Update and the like are half the size (to download).
- Will support very old 16 bit software (I use an old Win3.1 app without probs). Win7x64 will not do this.
AFAIK, there is extremely little 64 bit only software, so you are unlikely to run into probs with not being able to get sofwtare that runs under x32.