I believe that the upmix to 5.1 was done with the equivalent of a Dolby Pro-Logic II decoder, which is exactly what you'd hear if you played the stereo version through a PLII receiver anyway. The added benefit you get is the LFE channel that Hairy_Hen meticulously created.

However, you're right that it's impossible to know exactly what the LFE channel should sound like - the whole thing's a judgment call. But having heard it, it sounds simply amazing to my ears, and I can't imagine listening to the track without it.

Honestly, the only reason to use the stereo version (in my opinion, anyway) is if you'd be watching it on a stereo system that doesn't have a subwoofer. If you're using a surround system, I'd recommend the 5.1 version wholeheartedly.