The version type isn't a choice. It would depend on which type of windows you are running and whether the processor in your computer is a 32 bit processor or a 64 bit processor. You can find out which one you have by clicking the start menu icon, right clicking on computer and going to properties. The type of system will be listed there.

If you actually want to know the difference, a 64 bit processing system can technically handle larger sized amounts of ram and therefore "may" speed up the computer as it doesn't have to split the data into small chunks to process it.

Once you purchase the program you would be wise to go on subscription.  With that you'll have access to a 64-bit version if all you have now is a 32-bit computer.  In any event, AutoCAD and just about everything else you use will run much faster and more efficiently on a 64-bit box.  And you'll have access to more onboard memory as well.

Dave.