I have the HM-602 and the RoCoo P and I'm mostly listening to them with Westone 4 these days so I may be able to offer something to the discussion.
Firstly I would say that rumours of the RoCoo P being 'hissy' are, frankly, exaggerated. I don't notice any particular hiss through the Westones or indeed any other headphone, so I wouldn't worry about that if you are considering a purchase.
As for which player you will prefer that's very difficult to say. The HiFiMan 60x sound is pretty unique and is not to everyone's taste. It's dark, bassy, and somehow more "analogue" sounding than the average player. Personally I love it but I can understand why some don't. You're going to want very good headphones to get the best out of it - the resolution is extraordinary. As for the UI being "bad", it's not. It's just basic. If you want pet-sitting apps and HD movies then you're looking in the wrong place. Oh and the RoCoo's UI is even more straightforward.
The RoCoo P is of course much smaller, a bit cheaper, and with a longer battery life, and is at the moment the player that I take out with me most days (leaving the new Cowon Z2 gathering dust at home, which tells you something). I think it's a great little player with a really nice midrange presentation and no shortage of power (50mW per channel is quite something). I certainly prefer it to any of the big name brands' current offerings, and it's good value for money.
So, to the OP: If you can, have a listen to the 601 before buying if possible, as there's half a chance you won't like the sound. But if you do, it will put a big smile on your face. The RoCoo P you almost certainly will like, just as long as you understand that all it does is play music.