The L3400 is an older 34HP tractor, no emissions.
The L2501 is a new 25HP tractor no emissions, although quite a large motor for that power output (lots of torque, quite understressed).
The L3301 is a newer 33Hp tractor, with emissions.
All are pretty much the same frame size. The 3301 and 2501 are newer machines, I think there were a handful of comfort features that make them a bit nicer to use - but that really don't change the capability of the machine.
I'm personally not that concerned by emissions and I like more power, so at the same price and same hours I'd probably order them L3301, L3400, L2501.
A new L2501 v's a second hand L3301 v's an older L3400, I'd probably be tempted by the L2501. What altitude are you at? The L2501 loses power at altitude, and won't haul things up hills fast. But the loader lifts the same as all the smaller Ls do, and it runs a similar size bush hog. In my mind there's something to be said for a new machine.
To be honest, they're all excellent machines, you can't go wrong with any of them. More HP is always good. New is always good. No emissions if you can get it, useful. So each tractor has two things in its favour. Guess it's a question of which two of the three you value the most.