Well we don't have Guitar Centre in Australia, but last time I went to Billy Hide (Which is pretty much the same thing) they had only the Axiom plugged in and set up, so I couldn't have a feel of the sensitivity of the MPK pads. The Axiom pads were a little bit tough, if I do say so myself. The MPK is also noticeably larger and heavier than the Axiom.
I had a chat to one of the staff there. He said he prefers the Axiom because it's the midi mapping feature is actually really good. I'm pretty sure the last generation of Axiom didn't have a mapping feature or anything... I also asked him about the Axiom Pro. He said that they haven't brought out a second generation of it yet, so technically the normal Axiom is more up-to-date. He didn't seem too worried about the pads on either controller, because he had an NI Maschine. He couldn't seem to get over how great they are...
I personally preferred the feel of the keys on the Axiom as well. They seemed to be exactly in between weighted and synth action, which was nice. The MPK keys were really similar though. A little towards the synth action side, maybe - which isn't such a bad thing I guess.
Anyway, thanks everyone for your replies so far!