You should get 64-bit in almost any situation.
32-bit systems have a limit on the amount of RAM you can use (slightly less than 4gb), so it's silly to get it when 64-bit is the same price and is more future proof.
Something to note, if you are legitimately purchasing Windows 8.1 it comes with both 32-bit and 64-bit discs. The only reason you would want a 32-bit system is if you are having compatibility issues.