From a technical standpoint, I think Prokofiev 2 is a clear head or two above Rachmaninoff's 2nd concerto; not to say the Rach is easy by any stretch of the imagination, but his piano writing is a bit more comfortable in general. Interperatively, I find every piece to be challenging in its own way, so I'm unable to comment.

Here're some recordings I own or have heard (pianist, conductor):
Julius Katchen, Geoerge Solti - 10/10 - This is by far my favorite recording, although it doesn't seem to be mentioned often. I'm not particularly fond of Katchen's playing either, so I was quite skeptical when my friend sat me down to listen to this. Katchen sings beautifully with the piano, his rubato is unimpeachable, and his is the only recording where I find pianist and orchestra to truly act as one musical entity. I would recommend this recording before any others, as I couldn't imagine a better one. Solti is a great conductor too, by the way.
Artur Rubinstein, Fritz Reiner - 9/10 - This ties with a few others for second place on my list. Rubinstien has a no-nonsense, driving approach to the first movement which I especially like. It's just a great performance, and that's that.
Sergei Rachmaninoff, Leopold Stokowski (later version) - 9/10 - Rachmaninoff's tempo is indeed quite fast (although not quite to the same degree as his 3rd concerto tempo). His playing can have a certain sarcastic bite to it at times which may bother some, but his sense of rubato is unrivaled. His phrasing is also quite beautiful. The orchestra seems to fare a little worse, although this may just be the relatively low recording quality and mono sound which couple to make the orchestra sound a little lackluster. Still a great recording, and definitely a must for those who are playing the piece.
William Kapell, Fritz Reiner - 9/10 - A different take than Rubinstein or Rachmaninoff, but absolutely marvelous nonetheless. Kapell is one of few American pianists who really plays in an "American" way; has a very solid sound and solid approach, yet is always able to achieve a most delicate sound when the music needs it most. There's a great sense of depth here, in my opinion; I often finish listening to the recording only to wonder what small musical details I must have missed, for Kapell really leaves nothing beyond his care and thought.
Rafael Orozco, Edo de Waart - 8/10 - This was my first recording of the piece, so perhaps I'm a bit partial to it. However, I do think it's quite a good performance; I like Orozco's dynamics a great deal, and he plays in a non-sentimental way which other pianists often fall prey to doing.
Vladimir Ashkenazy (not sure which recording) - 4/10 - I'm less of a fan of Ashkenazy than most, it seems; I find his initial tempo too slow, and the orchestra lingers on far too many notes, producing a very sappy, sugar-coated sound. Ashkenazy at times seems to be asleep at the keyboard... I simply find the entire affair quite boring, and would not recommend it. Unfortunately, I'm not quite sure which recording of this I heard; perhaps another one would be better.
Sviatislav Richter (not sure which recording) - 5/10 - As much as I love the pianist, I really don't care for the recording. The first movement is terribly melodramatic. The third movement is really great though. I've only listened once, so I can't really say much more.
In the interest of time, I'll just cut the rest short and say Graffman's pretty good, Cziffra's pretty bad, and Kissin's in between.

- M
(Oh, and Van Cliburn's is good as I recall, but I am not very familiar with it so I can't really say for sure exactly how good it is)