I also have a 2520, and am also very happy with my decision to go with it. When I was looking to purchase, (Last August thereabouts), I was originally looking at the 2320, which in reality probably would have been enough for me.
I thought that because of the physical size difference between the 2305, the 2320 should have had a larger engine, not to mention that I was trading in an older Yanmar CUT, with a 27hp engine, and really didn't want to go smaller. The other differences that the 2520 has (faster hydraulics / Direct Fuel Injection / more weight etc.) sealed the deal.
Taking all the differences into account, as you say, between the 2520 & 2720, the only difference that I can see is the added hp. I have not had a situation yet, where I felt I needed any more hp. Certainly not for the $1800 price difference. If it were me, and I felt I needed more hp, I would be looking at the 3000 series. The 2520 is plenty capeable for what I want it to do.
As many have said, you will probably run out of traction long before you run out of power with the 2520, and adding a few more horses won't help that. Some have said, and probably they are right, the 2720 would run larger demand PTO implements better, but again, I think if you need more than what the 2520 will do, go for a 3000 series, and skip the 2720.
REV