Sometimes your hardware will decide for you. With the Mint 6 32-bit live CD, my computer rolled over on its side, kicked a couple of times, and died. I had to use "compatibility mode" to resuscitate my machine long enough to get an installation done. Once installed, everything was pretty much smooth sailing.
When the 64-bit version of Mint 6 was released, I ran the RC live CD with absolutely no problems whatsoever. I then installed the final version with the same flawless result. Did I need 64-bit? No, but my machine certainly did. As an aside, I can tell there is a slight enhancement in my video playback. My experiences with 64-bit have been extremely positive. Hopefully, if you need 64-bit, yours will be as well.