It's currently a 32 bit program. Though there will likely be a 64 bit "beta" version sometime within the next year or so.
Functionally, there won't be any real difference in program behavior. Windows will treat us differently, but just about everything should behave the same.
The main difference would be for large models where Windows won't allow you to solver for more than "X" number of load combinations because of 32 bit memory restrictions. That should be the main noticeable difference.