To be honest, I would just take the computer with the OS that is already pre-installed on it, mainly because there's less to deal with and honestly, the differences in 64 and 32 bit copies of Windows are pretty minimal with the exception that you have two installation folder directories now: "Program Files (x86)" and "Program Files." The reason that there are two installation directories is because now any programs that you would normally install under a 32 bit copy of Windows go into "Program Files (x86)" and any programs that are meant to run in 64 bit copies of Windows go into the plain old "Program Files" folder. For example, if you install Photoshop CS5, it will install Photoshop CS5 into the x86 folder and then install Photoshop CS5 64 bit into the normal Program Files folder. Plus if the machine you are buying has 4GB of RAM in it, and you install a 32 bit copy of Windows into it, you could end up hindering it because 32 bit copies of Windows can only read 4GB of RAM but use only 3GB of RAM. If the machine that you are buying has 3GB of RAM, then i guess its okay, but I'd still recommend that you not mess with the OS that is already installed on it, just to save time, effort, and trouble.
PS: Its rumored that 64 bit is better than 32 bit in the performance department as long as you have the proper hardware. I'd take a 64 bit copy over 32 bit in a heartbeat haha!
Hope this helps!