64-bit systems are faster than 32-bit systems. To check if your computer's 32-bit or 64-bit, on Windows 7, click the Start button, right click on Computer, and then properties. On Windows 8(.1), go to the Start screen, type "This PC", right-click (tap and hold on tablets), and select Properties.
TL;DR: Install the 64-bit version if your computer can handle it.