Mahindra 7520, Mahindra 3215HST, Case 580 extendahoe, Case 310 dozer, Parsons trencher, Cat D6,
All depends on your situation. The 33 will do everything that the 39 will do, just not as fast. The 39 will run wider PTO driven implements. The 39 will not pull any wider ground engaging implements than the 33, but it will pull that same implement faster.
So if you are not in a hurry to get things done, the 33 should be fine. But if you need-want to get things done a bit faster, then the 39 would be the way to go.
It's not often that you hear about people complaining that they have more HP than they need. My personal rule of thumb is to get the maximum HP in the frame size that I'm looking at.
I have a Kubota L3200, pretty much equal to the 3039 if I recall. I occasionally have maybe up to $1,000 of regret for not buying the extra 7 odd HP for a L3900. It would have been over $2k more when I bought.
Tilling really taxes HP. Mowing my fields is as often limited my rough fields as HP. Roading with any kind of hills is HP limited.
Low range is mostly limited by traction rather than HP.