I've owned both the UE's and the D600, but I kept the D600. The UE 6000 is nice, and a good deal for the price, but it was just too dark for me in passive mode (it sounded like everything had a veil over it). For hip-hop, the D600's are a good choice. The bass on them is top notch (better than the original D5000's even when modded, and rivaling LCD-2 quality), but the mids and highs are all funky. A lot of people like them, and a lot of others don't. I personally don't really love the mids/highs on them, but I would hate for that to influence your opinion, they're really cans that you need to try out for yourself to see if you like them.
On that note, I'm assuming from your name that you're in SF? If so, the best buys around here tend to have them on display in the fancy 'Magnolia' section of their stores so you can try them out (and they also have the UE 6000's on display with the other 'cheaper' headphones too). So you can just go there and try them both out in one go. The Best Buy on geary I believe had both in stock and on display last I recall, I only mention it cause I'm not sure if the one on harrison has the D600's.
Anyways, I just ordered a pair of M100's too, so I'll see how they stack up once they get here later this week. No idea about the other phones though.
I would say skip the d600s. IMO I feel the d5ks outshine the d600s anyday. You want a more neutral sound but with a bit of a thump which the Mad dogs will provide. The d600s would be way to dark for what it sounds like you want.
I actually live about 30 minutes south of the city, sons Best Buy with a Magnolia section is a little out of the way. But I look forward to your comparison! I think my final decision will be between the better of those two, the HE 300, and Mr Speaker Mad Dogs (If an e6 can power them, anyway).
Mad Dogs are great. Very neutral, but the bass is enough to keep me wearing them. VERY comfortable and isolate damn well too.