If sound quality is the only thing that matters to you and the um3x sound good to you then ill say,
GET THE DENONS, they are awesome.
I have the sm3 and IMO they are a slight microscopic notch below yet cost about $100 more.
I'm keeping both as denons is for home and sm3 is for on the go.
Lastly, I heard a decent of amount of full sized headphones (d2000, hd598, hfi780, dt770 pro, etc.) and a decent amount of iems (se530, fx700, ie8, ie7, etc.) so I will say that the denon's are the right choice for about $220 used. To me they sound a lot or up to a little better than all the iems I've heard. The sm3 comes close though but it is more expensive than a used set of denons. If money is no object and you may plan to use them on the go then definitely
GO FOR THE SM3.
To be honest either choice you wont be disappointed.