Quote from the Help:
»Faster performance with cross-platform 64-bit support
Complete day-to-day imaging tasks at least 10% faster on 64-bit versions of Mac OS and Windows. See “Memory and performance” on page 46.«
And a Forum search or some googling may turn up more …
From a practical perspective, besides the 64 bit version being a bit faster at just about everything, limitations on document size are lifted in the 64 bit version. Essentially, how much can do is limited only by how much RAM you have installed in your system. You can do things like have larger documents, deeper history, etc. and have it all work quickly.
On the other hand, many 3rd party plug-ins, especially older ones, only provide 32 bit versions, which will only run in 32 bit Photoshop.
Both 32 and 64 bit versions are installed by default. It's important to note that most of the preferences are separate between the two, so you can have two slightly different setups that might help with specialty operations you might need. This gets pretty subtle, but for example I keep the 32 bit version configured with only 1 cache level, so all previews of high bit depth images are always composited in high bit depth. This aids me with some aspects of astroimage processing. It's slower, but more accurate. By contrast I have more cache levels configured for my 64 bit version, so that's faster for general photography work.
Thanks Noel. Is it that "simple"? Can I open 64bit normally but switch to 32bit when I wish to use an older plugin that only works in 32bit? I have 8GB memory 2GB video and 2TB of HDD.
I have two icons on my desktop, one named "CS6 32 Bit" and the other "CS6 64 Bit", so I suppose the answer is yes, it's that simple. Only one can be running at a time, but each one will run just fine.