choppin is kinda tough in the mpc cuz the display is so limited. I actually timestretch in my DAW CEP 2.0 (soon to be pt le

)
and I chop using recycle. If you wanna use the mpc (no reason you cant dj shadow does) it will take some time to get the chops to sound "right", one trick is to chop drums and add blank noise at the end to get rid of that "robotic feel" the chopped drum loop has.
another is just "play" with the zones, even after I chop in recycle, I still go into zone mode and chop the hits down smaller, you can get some spooky sounding stuff this way.