32 bits, exactly.
Seriously, 32 bit Photoshop provides you the ability to install and run older 32 bit only plug-ins.
You may find your display driver does some things better in 64 bit and others better in 32 bit. It's rare to have that be the case, but I can think of one thing right now: I can only see OpenCL capability in 64 bits. This isn't really a "show stopper" issue, though, since Photoshop offers several backup plans (OpenGL, CPU execution).
I personally like the fact that they can be configured differently for most things and they remember their own configurations. For me this affords me the opportunity to easily start the 32 bit version to work on astroimages and the 64 bit version to work on general photography. In the former case, I open images through a 32 bit plug-in called FITS Liberator, and it's more important that while editing the document preview is composited with more accuracy, so I configure it for Cache Levels 1. Speed is more important for my general photography, so I configure the 64 bit Photoshop for less display quality, more speed.