64bit.....(Think the 32bit has a 4GB ram limit !!)
32 bit has a limitation where the total amount of RAM in your system (including graphics card memory and any devices you have that use memory mapped IO) cannot exceed 4GB, it will subtract from your system RAM if you exceed 4GB.
For most folk these days, 3GB RAM is the effective limit on a 32bit system.
OTOH, on a 32bit system, you get the 16bit subsystem allowing you to run ancient Windows 3.1 applications, this has been removed from 64bit Windows (but is still available under XP Mode on Win7 Pro+Ultimate). Also, if you have older or more obscure hardware it's more likely to work since some vendors still haven't produced 64bit drivers yet and in some cases the XP driver can work. You can also run unsigned drivers which may be useful if you're into rolling your own driver mods. This could however be considered a disadvantage because malware could install itself as a driver, something that isn't possible with the driver requirements of 64bit Windows.
32bit Windows also needs less RAM to function, so if you have a pathetic 1GB of RAM, go 32bit.
99% of hardware is working just fine on 64bit these days, so really it's down to which is more important, the 16bit subsystem or the ability to have more than 3GB RAM plus the added security of signed drivers.