Think the 2305 ROPS is probably fixed. This was the case with the 2210.
Think the 2305 has the hydraulic cooler fan underneath that can get damaged in rough terrain or when brush hogging even in smooth terrain.
Think the 2520 has position control and a suspension seat of some sort. It's also direct injection. It's also around $5k more. The upcoming 2320 is a closer tractor to the 2305, sorta similar to the difference between the 2210 and the 4010 and 4110. The 4000s were a little heavier duty but not in hp. Be about the same for the 2305 vs. 2320, when it comes out. The 2520 is more like the 4115, which also was around $5k more than either the 2210 or 4010.
There are pics of these, not side by side in here. Just search on "2305" or "2520" and you'll find plenty.
Ralph