Hey,
Major differences between the two:
1. The Roland comes with 64mb of PCM sounds stored on a chip, you can turn it on and make sounds with it immediately.
2. The Roland has more complex 'synth' parameters like filters, envelopes and LFO's.
3. The Roland has DSP effects.
4. The Roland has a BIG screen (I saw one the other day, it's about 6"x6").
5. The Roland can communicate with a computer via USB for editing and sample transfers.
6. The Roland can be maxed out to 288mb RAM, almost 8 times what the MPC can have when maxed out.
7. The Roland comes stock with 32mb RAM, which is 8 times more than the MPC's standard 4mb.
8. The Roland has a keyboard.
The Roland is CLEARLY a much more powerful machine than the MPC, I cannot think of a single thing that the MPC has over it...it's possible that the Roland cannot read sample formats like Akai S1000/3000 or EmuIII, that would be about it.
Take care,
Nick
PS: BTW, it would be difficult for Dr. Dre to get one of these on an airplane, that is another advantage of the MPC.