I liked the PODxt but I felt that I was missing out on what the POD HD has to offer. I sold the xt to help fund my wife's Ibby SR300 bass, so now I'm looking at either the POD HD Desktop or POD HD Pro (even though I'm far from one). I'll primarily use the POD for practice for now and once I feel I'm ready to gig, I can pair it with a powered speaker to do shows.

I'm not considering the HD300, 400 or 500 only because my back and my knees would not be able to take the constant bending and kneeling needed to dial in tones (I'm old).

My question is...other than various I/O's on the Pro (and the $300 price difference between the Pro and the Desktop), are these units basically the same?
Same exact software. Different form factor. The Pro might have more memory banks for presets or something, but I don't know for sure.

Having owned the HD 500 and HD Pro for a time, I preferred the Pro. It was just so much better on my back.
Ibanez Prestige RG852MPB
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Schecter Loomis NT
EVH 5150 III 50
PRS 212 DB
Line 6 POD HD500X
Deadhorse OD/Boss HM-2