The only difference is in the options you have for connections. In addition to 1/4" outputs, the Pro has XLR, digital outs, FX loop, unprocessed guitar out, etc. If you don't need any of that stuff, just get the desktop Pod XT. As far as comparisons of the models to the original Pod, I find the XT to have much more authentic modeling and speaker sims and to generally be much more like working with a real amp. The original Pod was much more plug-n-play, but after some tweaking, the XT definately sounds better to me, my sound engineer and to the guys in my band. If you don't have a good ear and a good idea of what sound you are going for, the XT is not for you. It is much less forgiving of bad playing than the original Pod, especially on the high gain models. I'm not saying I have a good ear or that I'm a great player, I'm just saying the XT requires a little more effort, but I think it's definately worth it.
I've recorded two CDs with the original Pod Pro and I can't wait to record with the XT.