I just went at a 36" white oak stump with my 391, 25 inch bar and half skip chain. It worked pretty good, but I still decided I wanted something with more power and found a used 066.
I bought my first saw about three years ago, a 290 and thought it had all the power I needed. Only After I blew it up, and bought the 391 did I realize I was using a under powered saw for what I was doing. I rebuilt the 290, knowing nothing about small engines, and now use it for smaller stuff. I realized my 391 is a little under powered for some of what I do, hence the used 066.
I would buy bigger than you think you need. Its an incredible feeling, after two years o struggling with an under powered saw, you pick up one with enough power to do the job and you wonder why you didnt buy bigger in the first place.
I was hesitant to go used at first, but looking back, I could have bought a used saws all along. The resources on this board can get you through any problems and there are some good deals from reputable folks on eBay or other places. For what you want to do with it, I'd save a little longer and/or find something used.
I would definitely rule out the 250. I do just firewood and can't imagine doing it with less than a 290, and I bought the 391 because I felt the 290 didn't have enough. Those are my thoughts.
