I went from a dr550 with a korg triton > mpc2000xl > mpc4000
I still got both mpc's, but once I got over the learning curve of the software it was just way easier to turn my ideas into something tangible w/o hunching over a dial all damn night.
software is definitely more portable.
I have my reasons for liking both as everyone else, but the 4000 is like falling into an abyss if you still at the beginner level like me.