Some folks feel the 25 PTO HP...as found on the 3320.... is adequate to run their implements. Myself included. However if you do heavy snow blowing and lots of HD PTO work the extra HP (as found on a 3520 and 3820) may be nice. It's typically thought that the compact tractors run out of traction before they run out of HP on ground engaging work (discs, blades, plows and such). FWIW
Another factor is that the 3320 does not have a turbo and a few more cubic inches. Pro's and Con's to both engines.....but I don't see a need for the turbo....just something else to make noise and a potential service issue at some point. (ok....I said it

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Myself? I'd rather put the extra bux into more accessories (grille gaurd, hydraulic outlets, HD bucket, etc) than buy ponnies I may not need.
Choose carefully.