Well, there is not many differences but here are a few (comparing the 2k-XL vs the 2k),
- 300,000 event sequencer vs 100,000 event
- 256 samples vs 128 samples in memory at a time
- folder hierarchy in file system vs "flat" file system
- receive on 4 separate midi channels vs only 1 midi channel
- dedicated buttons for pad banks and a few other features
- time stretch and resampling available on the XL
IMHO, go for the XL model. I highly doubt that Akai will improve the MPC2000 OS and will, instead improve the OS of the XL model.
If it wasn't going to cost me so much to upgrade to an XL model, I would go for it myself but I already own an MPC2000.