All really good choices...
All I can say is go to the local music store and listen. Then pick one.
You tried programming new sounds into MOTIF? Plus you can add sound cards to the MOTIF. I'd max out the MOTIF and MPC first.
What I'd do on a MOTIF and MPC setup:
Motif --> max. memory and some kinda expan. soundcard
MPC--> full 32mb memory and SCSI zip drive.
The ZIP drive-->2 50 to 25 pin scsi cords= 1 for MPC+1 for MOTIF.
Hook both the MOTIF and MPC to the SCSI zip so the can share samples. Use resample in the MOTIF to save EFX and phrases to zip and open them shits with the MPC. MPC for drums and Sequencing any additonal percussion, background, etc.... Motif for basses, key, instrumments, etc...
I'd go into the into the Motif and create a coupe of new BASS patches by taking existing basses I like and just select a sine wave instead of the internal bass sample. Tweak it a little. Save as a new patch.
Hope this helps ya out some.