I use 64-bit iof I'm only running Photoshop. The 64-bit will use most of your available RAM. So; I use 32-bit if I need to use any other programs & manipulate images on the fly. I found some more info from Adobe; see below.
Photoshop 64-bit OS benefits & limitations (from Adobe)
Photoshop CS5 and CS4 install a 32-bit and a 64-bit version when you install on a 64-bit version of Windows 7, Vista, and XP. If you are using the 32-bit version of Windows, then Photoshop only installs the 32-bit version. Photoshop CS6 allows you to choose one or both of these versions to install. Photoshop CC installs both versions. Photoshop CC (2014) and later installs the 64-bit version on a 64-bit OS by default. You can download the 32-bit version here.
Photoshop installs a 32-bit and a 64-bit shortcut into the Start Menu. If you need to manually run the application, the 32-bit version is the photoshop.exe file, in the Program Files (x86)/Adobe/Adobe Photoshop [version] folder. The 64-bit version is the photoshop.exe file in the Program Files/Adobe/Adobe Photoshop [version] folder.
RAM use (Windows)
The primary advantage of using the 64-bit version is to access amounts of RAM beyond what Photoshop can access when the 32-bit version is run. You can take advantage of more than 4 GB of RAM only when you are on 64-bit systems, using 64-bit Photoshop. If you use files that require more than 4 GB of RAM, and you have enough RAM, all the processing you perform on your large images can be done in RAM. It's not necessary for Photoshop to swap out to the hard disk.
This table lists the amount of RAM available to Photoshop with the different versions of Windows:
Photoshop version Windows version Maximum amount of RAM Photoshop can use
Photoshop version/Windows version/Maximum amount of RAM Photoshop can use:
32 bit / 32 bit / 1.7 GB
32 bit / 64 bit / 3.2 GB
64 bit / 64 bit / as much RAM as you can fit into your computer
Third-party plug-ins for 32-bit versions of Photoshop don't appear under the Filter menu when you run the 64-bit version of Photoshop. If it's necessary to use plug-ins that haven't been updated, run the 32-bit version of Photoshop. When you are done using the plug-ins, close the 32-bit version. Then, run the 64-bit version. Contact the plug-in manufacturer for information about any updates.
Processor speed and Photoshop operations:
Although the 64-bit version of Photoshop speeds up some operations, it doesn't speed all of them. And, it doesn't speed the operation equally. Generally, operations run approximately 8-12% faster. Overall, processor speed is not the main advantage of using the 64-bit version, except when you run actions on large files. Photoshop writes data to the hard disk incrementally while you pause in Photoshop. When you run actions, there are no pauses for Photoshop to write to the disk. Therefore, Photoshop writes to the disk while the action runs. The increase in processor speed increases the speed of these processes.