First a disclosure: I've got an MPC2000 Studio w/ all expansions that I will sell you real cheap. Reply to this post and we can discuss it further.
Now here's the thing about the MPC vs the SRX board - The MPC can load any .WAV sample you want. Second, with the expanded outputs, you have 10 outputs. Many people like to EQ each drum differently, so that's where having the extra outputs is cool vs the limit of 4 outputs on the Fantom-S. Also, alot of people like the feel of the MPC sequencer - that's why they're so popular for hip hop and dance music. Finally, the way they allow you to switch between record, play, and overdub is very cool when you're "in the flow". And the 16 velocity and pressure sensitive pads are cool also - especially the 16 velocities feature. Although if you have a Fantom-S, you have a facimile of that. However many people complain about the response of the pads on the Fantom-S, which is assumably why Roland reworked them for the upcoming "X" series.
So why would I be getting rid of the MPC? Because I want to move into computer sequencing, and the type of music I want to transition to does not require the MPC. However if I can't sell it I will happily use it for the 16 velocity sensitive pads and to trigger extra drum sounds.