Bootcamp: is there an advantage of 64-bit vs 32-bit Windows?

According to the store pages, it looks like all the retina models have 8GB memory, with option for more, and as such you should choose the 64-bit version to make sure you can take advantage of all that memory in Windows.

A 32-bit Windows installation would only be able to use 2GB/3GB of the available memory.

Also the bootcamp drivers for certain Mac models are only compatible with 64 bits versions of Windows, so first check the bootcamp drivers versions! To do that, go into the drivers folder of the bootcamp package, then open each sub-folder and check the naming of the files: if they all end with 64 then they can only be run on 64 bits versions of Windows.