The 2650 weighs more, and has a bit larger rear tires - both good for traction. It has just a touch more HP at the PTO. The FEL has a bit more capacity, but that isn't very noticeable of a fairly light tractor - either one can put the tractor on its nose. The 2650 has a higher capacity on the 3 point hitch, as well. And it costs about 20% more new.
The 2620 has a two speed PTO, which can come in handy at times; like when you are mowing light grass and don't want to race the engine, or tilling and want a finer cut. Its mid-pto is optional - make sure it is there if you are buying used, and eventually want a mid-mount mower.