I had both the MK III Sniper in .25 and the Compact (in .25, then switched to a .22). Ergonomically, I loved them both, but the Compact was definitely the more handy package to walk around with. My .25 Compact was giving me 55 FPE using 33 grain JSBs as it arrived from Ken Hicks at SPAW. In .22 it was a lovely shooting gun, and deadly accurate out to 50 yards (as far as I took it). Apart from the higher shot count I can’t see a reason to choose the Sniper, assuming you have other guns that can give you more power when you need it.
I sold the Sniper because I was buying a bunch of semi-auto airguns, and because I had other guns (Impact, Crown) to give me more power when I wanted it. I eventually sold the compact to help finance my Leshiy 2, Pro X and K1 purchases and because it duplicated what i had in my .22 Verminator MK II. I am currently considering getting another one, because I miss it.
Chris