Well, I went from a 2305 (that didn't have) to a 2520 (that does). Before I upgraded to the 2520 I too was doing my homework and I thought the biggest feature was the ability to return it to a previous height setting, which comes especially useful when grading, snow blowing, etc. What I discovered was that this return-to-previous-setting feature was really nothing more than an adjusting bracket beside the 3ph lever beside the operator. If you want to adjust the return-to-previous setting, you loosened the wingnut, moved the bracket to where you want it and you re-tighten the bracket, and voila.. new height setting. This helps you do what Chris describes earlier, back and forth passes with a blade at the same height.
However once I started using the new 2520 with position control, I noticed one other huge difference (which I really really like, btw). The 3ph lever on the 2520 controls the EXACT position of the rear 3ph, meaning, you inch the lever forward on the slider by a little bit, it raises and little bit.. you inch the lever on the slider a lot more, it moves up a lot more. Meaning, the position of the 3ph height is relative to the lever position. This comes in especially handy when doing finishing grading, when you're trying to move dirt to strategic spots, raising/lowering particular areas with material, etc. etc.
On the 2305, the 3ph lever basically had three settings. Up, Middle and Down. When the lever was in the Up position, the 3ph is being raised.. when it's in the Down position.. it's being lowered. When the lever's in the Middle position, it's stationary. So getting the exact height you want is a matter of constant lever movement betweeen the three settings until you get it right.
I've done a lot of grading and material moving with the new 2520 and I can honestly say it's a very effective system for controling the height of your rear implement. If you plan to do a lot of work with your tractor, I think this position control feature allows you to handle the implement with a little more accuracy.
Hope this helps!
GordNovo