I think with that kind of wear time, comfort needs to be near the top of the deciding factors and I love the M50's, but I could not see wearing them for more than 4 hours a day. In comparison, my less expensive Panasaonic HTF600's which are about a 1/3rd of the weight(build quality too) are like having a feather on your head-those I could see wearing a full work day if I had too.
I think best bet is to narrow it down to a couple choices, buy both on Amazon and return the one you don't like.
Comfort is probably one of the more top things on my list just because I do use them a lot and it seems that I will never find the comfort of the Uptowns I had in the M50's/HFI-580's but I can deal with having to take breaks away from em for 15 minutes and resting. As far as the buy both thing, I was considering this but idk, being that I am going to be amping them I cant afford both and still be able to amp them hence it wouldnt be a true test of which ones I like better, just an idea, but I am still considering doing it even then.