Will these be for walking around, or just something you can take with you? Open headphones aren't the best for walking around, you'll share everything you're listening to with those around you, and you'll really have to turn up to block out environmental noise - furthering the first problem. What sound characteristics are you looking for? M50's have a slight V shape, 598's are a warm, mid-centric phone.
Most of the time I'll be using them at home and it's not really noisy or anything, but every so often I'll need to use them around like 15 other people where it is pretty noisy, so I guess I'm kind of screwed there.
And I don't really know the terms to describe sound characteristics that you're using, but like I said, bass is good and I mainly listen to music like house and electronic in general.