First duh answer would be "mpc2500"............but then i would say it depends on exactly how you have your mpc incorporated into your setup and what you are sampling and how you use the sampler.
The motif sampler is less user friendly compared to the mpc for general sampling needs. however, that doesnt mean its bad or not as feature rich. if you loading keys/keymapped sounds and making keyboard style beats already and just need the sampler to extend that, then it may be useful/better to keep all that shit inside the motif, especially if you are just using the mpc as your sequencer and not loading sounds into it.
however
if you chopping loops, drums, breaks, vinyl, etc. then no doubt the mpc will win. its easier and quicker to chop and more condusive to that workflow for creating kits with hits, stabs, loops, etc.
mpc2500 + jjos2xl can do chromatic pgms (for keymapped style samplles) however, its not as feature rich in that department. I think it also depends on your familiarity, you might find the motif to be great - some people do
in my personal opinion, i'd say **** the motif for sampling. just use the mpc.but this is comming from a dude who does east coast style sample based beats (dj premier style, etc.) so the only reason i have a motif is for those extra synth sounds like basslines, and occasional synthy sprinkles.........i'm not into making keyboard beats and have no motivation to build a key/synth sample library.