I've owned both headphones. I returned the Momentum and kept the 1R -- a decision that came down to comfort. The Momentum ear cups were too small and I found myself constantly having to readjust them to alleviate pressure points. The 1R, on the other hand, is the most comfortable headphone I've ever worn. With its cushiony and spacious ear cups, as well its light weight and gentle clamp, they disappear on my head. The only potential comfort issue with the 1R is that if your ears stick out far enough, they can touch the driver. So, I recommend trying these on first just to make sure they don't.
I enjoyed the sound on both headphones. I feel like the Momentum is the more energetic can, while the 1R excels with vocals and soundstage. I think I slightly prefer the 1R sound signature since I listen to a lot of vocal trance and other electronic music, but both sound awesome.
I have to give the Momentum the edge in build quality. This is the only place where I have complaints with the 1R. A common issue with the 1R is jack rattle caused by a loose cable connection at the ear cup. Actually, the connection itself is very secure, but the inner housing of the jack in the ear cup can wiggle. Some users have applied blu-tack as a fix. Additionally, the 1R is prone to creaking during head movement. I've gone through 3 units from Amazon, and 2 had the loose jack issue. The one without the loose jack had some minor cosmetic defects. Go figure. Another replacement should be here tomorrow.
Although the Momentum is smaller, the edge in portability/transportability goes to the 1R. Since the 1R's cups swivel, they can achieve a flat profile for easy transport in a laptop bag. The Momentum comes with a hard case, but it's way too big to be useful.
If the Momentum had larger ear cups, I would've just stuck with them. I've demoed quite a few headphones over the past few weeks, and the only reason I'm willing to go through the hassle of getting the 1R replaced multiple times is that there is nothing else that has this perfect balance of comfort, sound quality, and portability.
Edited by acti0n - 3/19/13 at 10:06pm