I figure many people in this topic area will be looking at MIDI controllers as they evolve. I just finished testing both the newer "Oxygen 49" and the "Axiom 49" MIDI controllers from M-Audio. I kept the Axiom after some deliberation, and considering the small price difference ($50 more for the Axiom). These two boards are similar but there are important differences:

(1) The Axiom has 20 memory locations to retain settings -- the Oxy has 10.
(2) Axiom has at least 4 key "zones" for layering and splits -- Oxy none.
(3) Axiom only has 8 rubber "drum pads" for more precise control of percussion and other effects.
(4) Axiom has an additional output ("espression pedal") and apparently a dual channel USB MIDI input to the computer -- Oxy has no extra pedal I/O and 1 computer electronic connection. Both can be powered in USB mode from the computer connect alone (but not through an unpowered hub). USB 1.1 specs.
(5) Axiom has "semi-weighted" keys -- Oxy the regular synth type. Both have aftertouch, however.

This last appears to be more important to some than others. I have a hard time seeing much difference -- this may change after breaking it in. The Axiom in its more substantial case (10 lb. versus 7.5) looks more "pro" and durable, though this may be an illusion (not the first time, if so). I believe most of the MIDI controller functions are almost identical. Both series come in 25, 49 and 61 key versions. The smallest lose a few controller knobs, etc. and perhaps something in the I/O group.

After really trying to use this new KB for a while, I'll comment on its actual utility. We may here from others who are also picking up these units.