The Roland MV8000 is a linear ONLY based sequencer at the moment, so if you want to break your beats down via patterns ( as the mpc does ) you cant. FX is very, very basic too. They are coming out with a video card option, that will alow you to hook up a computor moniter and mouse and edit and control the MV8000 via the moniter ( good idea IMO ).
The MPC 4000 is a great unit, it has some bugs, and akai support is very questionable ( they are having some finaical issue's you could say ) but still a good machine.
Another thing, both have a single set of stereo outs as standard, MV8000 has 2 midi outs, MPC 4000 has 4 midi outs.
Like its been said before, research them both, mess with them both and make your choice off what works best for you.
Hit up
www.rolandus.com they have a free dvd you can order that shows the MV8000 in action. The cat that demo's it is a midget I swear, he makes the MV8000 look huge
lmfao.....