2 years on my L2501 now. The differences in the 2501 and 3301 are very little. If I was going to upgrade above the L2501 I would go to the L3901, which is a more substantial jump. The one thing to keep in mind however is what are you actually getting between these 3 models. They have the same hydraulic pump, same loader, almost identical lift capacity. So L2501 will lift just as much weight as the 3301 and 3901. They all 3 weigh within 100lbs of each other, and are built on the same exact frame. Physically the tractors are the same size. So the main difference you are paying for is the bump in horsepower. Given the weight is the same, I doubt the small extra bump in horse power means you will pull a lot more weight though. Your tires will start to slip in most all scenarios and that will be your limiting factor on pull power - the need for more weight rather than more horsepower. The main highlight of upgrading, is the PTO horsepower bump. This will allow you to more comfortably run a 6' mower for example, or a 6' rototiller. My WG24 pto driven stump grinder for example would also benefit from a little extra pto horsepower. This is the major gain to upgrading. If you dont plan to use the pto a lot, then upgrading has very small benefits. While ive not heard of any issues at all with the DPF on the 3301 and 3901, it is nice that I dont ahve to deal with it at all on my L2501.
Things I have done with My L2501 just so you know: Cleared an acre of woods so far and converted to grass. This involved a lot of very heavy log moving from downed trees., running a 72", 800lbs disc harrow on the ground to break and chop it up. Grinding all the stumps with the Woodland Mills WG24 PTO stump grinder, running a 60" rototiller (6 tine, forward rotating) to seed grass and in my garden, etc. Move 5' hay bales using a 3pt spike (probably 800-1000lb bales). The L2501 is one of the strongest 25hp rated tractors you will see in the market imo. It outclasses the small 25hp machines by a long shot. Dont hang up too hard on the HP rating, as it is only one factor. The size of the engine is important. the weight of the tractor is very important. The L2501 has a large engine, in many cases it doubles the physical size of other 25hp tractor engines from smaller frame models. It has good low end grunt and torque.
You are going to have a hard time finding differences if you just test drive the 3. A good partion of all 3 tractors are the exact same parts. If you plan to use the PTO a lot, meaning mowing a lot, tilling, etc, I would jump up for the extra pto horsepower. The only time Ive ever wished I had just a tad more on the L2501 was in grinding stumps on that stump grinder. I feel it would power through just a tad better with more horsepower. Very few people are running a stump grinder on the pto though. If you just plan to run 5 foot implements, the L2501 handles them fine. If wanting to run 6 foot implements Id want the extra pto horsepower. Best of luck!