(I'm currently looking for my first midi controller so half the stuff I say is probably wrong. Please correct me...)
Had a look at the MPK and Axiom in store. I was surprised by the MPK's weight. It doesn't *look* all that heavy but it is. Felt like 3 or so kilograms. I dunno what that is in imperial... Anyway - weight, for me, usually means solid build. So I try to go for heavier thing because they are usually more solid and can take a beating but from what everyone on this thread has said it seems rather flimsy. Which lead me to wanting the Axiom. But the axiom has it's own set of problems that include keys that stick down when pressed, wibbly knobs etc. Also it doesn't have an inbuilt arp (to the extent of my knowledge), whereas the MPK does, as mentioned above. I don't think the arp really matters to me though. I mean, I love arps, but you can equip an arp as a midi effect to any vsti in ableton and I wouldn't be using the controller outside ableton.
On anther note, I could always just scrap all these options and get a Maschine. That thing looks friggin awesome. No proper keys though...