Prokofiev Concerto 2!
The second is the better concerto, and also considered the most difficult, I believe. If you are able to play it, play it.
You should always go in order. For example, I hate it when people skip all Chopin ballades and go to the last. I think you should go for concerto no. 2 now, since you have 1, Maestro.
I dont know of any pianist who would consider the 3rd harder than the 2nd. Not only is the 2nd longer, but it is also taxing in ways that the 3rd is not. The 2nd mvt of nr 2 is an all-unison moto perpetuo with both hands playing 16th notes non-stop - the very few intances of anything similar in the 3rd (like the recap in the 1st mvt, and thats not even completely in unison, and besides, completely pianistic), it usually lasts just for half a minute or so. In this sense, the 3rd has many of the same challenges but on a smaller scale - compare for example the final movements of the two concerti, which are similarly structured, with fast outer sections and a slower lyrical middle section. The 3rd concerto has a somewhat "compressed" middle part, while the 2nd has a very expansive middle part. And let's not forget that the 2nd concerto has its difficult cadenzas - the only solo you have in the 3rd is a solo in the 2nd movement.That being said, the 3rd one poses certain problems as well, especially in terms of balance with orchestra and sense of continuity. I've studied and performed it (with 2nd piano only so far), and with so few moments to relax (the pianist is busy almost non-stop for half an hour), it is still a taxing piece, in my opinion. Go for the 2nd if you dare, do the 3rd if you dont