I'll be going entirely against the grain and say Shostakovich. I have played both in their entirety, and the Shostakovich gave me more problems technically. Of course, the Grieg is another ten minutes longer, but once you learn the notes, getting it up to speed was a cinch for me. Also, it's one of the most pianistic concerti I've ever played. It fits into the hand really well, and is even easier to memorize. Shosti, especially with the Sightreadable slow movement, looks sparse and simple, but really, it's much more awkward and unpianistic. In the overall scope, Shostakovich is harder to master, but not by much.