For portable, I find Momentum & MDR-1R makes the best all rounder although I never get to try the P5 but here's what I think about both Momentum and 1r:
Before I get started, I find both are great looking headphones when it comes to design/fashion.
Momentum:
These earpads & headband are made of leather, isolates plently of noise if you have M-size ear as the earcups are too small for me. It is more like a on-ear for me. As for sound I find these overall bassy & warm sounding. The lows are bassy, punchy but lacks tightness. Mids are warm sounding overall. Where as the highs are clear. Details are good but when it comes to soundstage, I find Momentum are lacking in it. I feel like instruments are all squeezing together. Transparancy are excellent overall. It's very very clear with minimum grainy sound.
MDR-1R:
These earpads too looks like it's made of leather or pleather I'm not so sure
. Comfort are better on these than Momentum as it's earcups are larger & more flexible. The sound signature on these I find it like others mostly said. Flat. The bass to me are less than Momentum & better controlled which is a good thing to me. Mids are natural sounding where as the highs are clear. Soundstage & details are better one these than Momentum. These two has good transparency but slightly behind Momentum. Fortunately, there's no sibilance/grainy sound detected during my listening.
Lastly, I am not sure if these two are gender to bad records like mp3 file format as I did both of my listening with WAV. Momentum with my iPod Nano 3rd & MDR-1R with my iPad as I audition them at different stores.
Just my 2cent. I am too very interested in how people find these two to sound like