As far as the sequencer, there's not much difference. Same time sigs, edit capabilities, timing corrections, Loop, and metronome stuff. I do, however, wish they added TC values larger than 1/8 notes, like 1/4 and 1/2 notes. I never used the metronome much, but I think they added more options for what you want to use as the sound. You now can choose your own samples for the metronome. One sample for normal and another for accent. This is much better than the old annoying click.
TRACK NAMES. You can now define default track names for newly created sequences. The track name is still carried over when you copy sequences, which is probably what most of us do anyway and would be more helpful for people you write new additional sequences from scratch. One note, I'd like to add here. Most all the information I will discuss in this message is saved with a sequence or 'all sequences' file. A lot will be gained if you setup you machine with default values and save it for you initial load each time so you don't have to re-enter that data again.
MIDI IN AND OUT. Much has changed here. Akai has improved how the MPC handles data in and out. This really will not concern you if you do not use a master controller style setup. I personally use a K5k as a master controller, though, these new things are now available, I may not even use them. This is probably easiest if I divide into 2 parts: Midi in and Midi out.
MIDI IN. There are now 2 ways to record midi data from an external source. You can record a track into the selected track (the usual method) or do multitrack midi record. This is the difference between the S and M on the lower left of the screen. What ever you set this for, will stay that way for every track and sequence you select, used or not, until you switch it. So it can be called a 'global' setting. Single track is the usual: whatever notes come in get recorded on that channel and the midi data *may* come out the midi out(s) on the midi channel that track is set up for (I'll get to the *may* and out(s) in the midi out part later). For multitrack, you OPEN WINDOW for the multitrack setup from the 'M' and set each incoming channel (In1-Midi1, In1-Midi2, In2-Midi5, etc.) and the destination track you want each of the incoming 32 midi channels to end up. That means you can record 32 simultaneous midi channels plus 2 channels of exclusive on to 34 tracks. Did I say exclusive? yep, I did. You can record system exclusive data on its own track. Two tracks are for this since you get one track for exclusive data from each of the inputs. Along with the track assignment you get a destination output for incoming midi channels. This part, in actuality, is same as assigning a midi channel for each of the tracks in the usual method. If you setup the multitrack and then go look at each of the tracks you will see the midi channel settings for each of the tracks.
I know that might have been sort of complicated, so I will summarize the above here: On the former model, you could only record 16 midi channels at once because the MPC treated the 2 midi in's as one (except for sync signal). So now, Akai added this multitrack screen, that basically allows you choose which input (1 or 2) to record from. This just doubles the number of channels to 32 that you can simultaneously record.
MIDI OUT. Small changes here. The usual is to output on whatever midi output is set for the track. This is called the soft through function. (Short explain here if you don't already know: your master controller keyboard usually sends midi out on one channel [which channel is up to you], the soft through takes that midi data and outputs it on the midi channel for that track. The idea is you have one hand on you controller and another on the track selection buttons and as you change tracks you are actually changing which midi modules get midi data. other similar uses too). The additional setting you can make here for each track is you can set the track SOFT THRU (as described), OFF, OMNI A, OMNI-B or OMNI-AB. The OMNI setting just passes whatever midi data straight through the MPC on the same channel it came in on with the choice of going out A, B or both. This additional small feature I can't see getting used a lot, by me anyway. I just use the soft through feature.
MIDI MONITOR. This is another new feature. You can open either the midi in monitor or the midi out monitor, mostly to double check how you midi routing is doing. You can't go into this screen while sequences are playing though.
DELETE OFF TRACKS. Now that there's a dedicated TRACK MUTE button, the old location to get to the this screen has a different feature. When you open window on the 'track: on/off' spot on the main screen you get a popup window that ask if you want to delete all tracks that are turned off. This is very helpful. Why? I like to create the meat of my songs just using one sequence, then copy this sequence many times with different variations of tracks being on and off. I would use these sequences and chain these sequences together in the MPC's song mode. Once that is done you can convert this song into one sequence for final editing (and recording all that CC data). Well, on the former MPC, if you did not manually erase the tracks that you turned off, the note data of the tracks that were turned off would show up in the final sequence from the converted song. This 'deletion of muted tracks' keeps that from happening and should cut 10 minutes off of my 20 minute song process. :)