The 3000 series tractors are very capable. You need to know how much power you are going to need. I have a 3320 and I can spin all four wheels if I try to pull too much of a load so it has plenty of power. It runs a 6 foot flail mower without a problem. It has no problem with the 59" snowblower.
The reason for stepping up to the 3520 or 3720 is because you need more pto power for a specific implement, usually that would be a larger mower. I ran my 6ft tiller on the 3320 preparing 1000 feet of garlic row this fall and it had no trouble at all. That said, I have nice soil too.
It's a great small utility tractor. If I was to get a cab, I would probably push to the 3520 to run the A/C in the summer and still have the same pto hp at the rear.
