Hi 3500,
I have had both tractors, first the 3320 now the 3520 and liked them both.
The 3520 actually feels less powerful until you get the turbo spinning (2000+ rpm) in my experience--don't know hoe torque/hp curves match that though.
Because of that I preferred the 3320 for loader work & using the box on the back, as It seemed to be as capable at a few hundred less rpm (for the ears to deal with). Both lost traction before they ran out of power though.
Using the PTO though, the 3520 has noticeably more grunt at PTO rpms... the cutter and tiller don't bog down as readily, and you can set the slip clutch a little tighter.
If you plan on a cab the turbo will be needed to offset the increased heat/AC load on the motor.
Good luck! Both are nice machines.