I compared all three, 2305, 2320 and 2520 side by side at the dealer. The 2305 is a great tractor but I wanted more ground clearance and I did not like the hydro filter and fan being so exposed underneath. I go into the woods and I'm sure I would have ripped the filter out and/or busted the fan doing that.
The 2320 gives you a lot more clearance than the 2305 and is exactly the same as the 2520. The real differences are 2.5hp, and somewhat quicker hydraulics on the 2520. The only real benefit, according to my dealer, and I tend to agree is that if you want a 72" MMM then you have to go to the 2520. Other than that the 2320 has the same ground clearance and same 3pt hitch capacity as the 2520. The increased expenditure was not worth it because I did not need or want the 72" MMM.
My 2320 handles the 62" MMM and 61" bucket great. The deck is so easy to take off and put back on. I don't think the 72" deck is a roll over, is it?
All 3 are great tractors, very well built and should last a very long time.