honestly? keep the mpc and get yourself the rack version of either the fantom x or motif.... why?
i have worked with the following:
triton le, motif rack, the roland xp 80 and 30, the proteus 2000, and the tr rack.
the triton, while admittedly a scaled down version, still isnt worth the money that it costs, at all. the earlier roland boards were superb and i left the 30 for the triton le which was a HUGE mistake, and i got rid of the 80 to get the motif rack... by FAR the best sounding of any keyboard... except for the fantom patches i have heard on the x. and those are really a toss up.
i say go with the fantom xr modlue- if you have the mpc you you dont need the additional sequencer, and the mpc sequencer is the best and easiest i have ever worked on with the exception of cubase sx( i now use cubase sx and sampletank 2, in case you were wondering.) and i would kill for that fantom x or the motif rack again...
it really isnt worth it to get the keyboard versions- your mixdown options become very limited, and the total of 18 outs with an mpc and module combined are invaluable. i say keep the mpc ad get a rac moudle and midi keyboard.