for a mpc you upload a song you want and you can cut up each part you want onto it.
for a mpd you hook it up to a computer and cut up songs on the computer and load them onto the mpd, it's pretty sweet and as you see you can make some sick beats with it!
More then likely it has to do with memory/effects/sample time. My bros got a 2000xl and a s950. The s950 has ill ass punching drums but has like 6 seconds of sampling while the 2000xl has mad sampling. Also u might need the oldschool ass wide mac floppys for saving sequences.
Looking for original blends... peep my myspace (In my profile) or hit me up. I got ones that will make your edits hella dope.