i like both machines. i dont think one is better than the other one just does more. If i had to pick between the two Id probably take the 5k. I like the bigger screen and the lay out.
Work out what features you will need and see which one matches you best. The 5000 looks like a beast, I mean its even got its own synth in, but it has had bad reviews.
I'd try and get a used 4000 instead of those two personally.
These new mpc's are getting worse not better. I had a 1k and the pads sucked. Then the 2k which freezes when i'm in the middle of a groove. I have the 3k now, and so far it has not let me down. Plus, every function is at the touch of a button. That helps me stay in the groove. I wouldn't get any newer mpc that is made in china.
That joint will weed out the weekend warriors from the real heads.
Talk about stretching ability. 13 seconds, 5 second max samples. Mine has done nothing short of turning me into a beat making macguyver, trying to stretch it's limitations to the fullest ahahah.