Overall signature for the Q40s is brighter (i.e. brighter upper-end and mids), the attack on the M-Audio's is also faster (probably perception due to the added air up-top). Mids are a bit more recessed on the N-Pulse's but I felt the bass to be bigger (mid-bass especially but sub-bass was tastefully present as well). I feel the N-Pulse's to be a bit more "ear-friendly" as sometimes the highs on the Q40s can be a bit much on some tracks to me. So basically the Monster's are a bit more laid-back due to their bit of warmness across the whole spectrum. Soundstage is actually quite on par but just like the Q40s on busy passages (especially when listening to metalcore or similar genres) it feels more closed in than usual. Comfort can go either way. I feel the Q40s clamped too much even when "stretching" the headband for a bit but with updated pads it felt better. The Monster's on the other hand don't clamp as much and earpads feel quite good.
So for the price I got both (let's say $120 for the Q40s with Beyer pads and $95 for the N-Pulse) they are a steal for any basshead looking for very good bass, slam and overall sound. I've seen the N-Pulse's go even lower used on eBay (and I don't think there are any fakes yet since these were released not too long ago and I don't think they are as famous as the Beats) and you can even get them cheaper on amazon: http://www.amazon.com/gp/offer-listing/B009IKCKMY/ref=dp_olp_used?ie=UTF8&condition=used
I'd pick the Monsters just because I've been liking "warmer" signatures and find them comfortable and pleasant enough. Being cheaper "used" doesn't hurt either. ;)
P.S. Impressions based on memory. I no longer have both on hand.
What do you mean by warmer? i hear that a lot aroujnd here but idk what it means, please ELI5. Ill really try to see if i can try out both, hopefully best buy has a demo out on the floor to test out. im getting excited that im getting closer to my goal finally and i actually feel sure about these choices!