I recently purchased and MPC 2000xl and attempting to learn this machine. As a novice I can't give a complete comparison but so far these are some differences I noticed.
1) Sampling / procedures with the SP 404 are a lot quicker than the MPC
2) 404 has WAY more effects than the MPC. This seems odd aks the MPC cost WAAAY more than the 404
3) This is a personal one, it seems that you can work faster on the 404 than the MPC, again I'm a novice so this may be why.

Many have said that once I learn the MPC I'll be able to create faster which I'm sure of. I wish the SP 404 had the edit window of The Sp 505 or 606. Maybe they'll do that with the next model. I was suprised that the 555 didn't. I'm interested in hearing peoples thoughts / experiences with the MPC. Thanks.

yeah man, I own the 2000xl too. The workflow isnt as good as a SP404 but I like the workflow on it for an MPC. I dont like the newer MPCs, they seem to have that thing where you hit shift and then hit the pads for the functions.
The 2000xl is more of a professional machine than the SP, it has more than I need. I'm good with just the 404.