I think there is no valid comparison between DS and JR, both are great in their own way.
Derek has a more rock background, and that comes out in his music, sounds and way of playing. I still love some of his parts on Metropolis pt.2 demos, very alien and different sounding. He had (and has) more edge to his sound, he is more restrained and more limited in sound range.
JR is a wizard when in comes to tech, coming up with new sounds, but he has also a classical background, more prog-oriented and he doesn't try to restrain his playing, sometimes overdoing it in my opinion. But if you take the LTE albums and compare them to Octavarium, you can see how much of an amazing composer and sound designer he is. He has an incredible taste in sounds and given the proper setup, he can create amazing things. He sometimes can become a bit cheesy but still i love his playing. I think Octavarium and The Astonishing are his best albums with DT.
Still, KM is my favorite even though he might be the least diverse in terms of sounds
