I've been happy with my Sony MDR-V6, which I think are almost identical the the MDR-7506. $100 on headphones seems pricey if you're used to earbuds, but these sound great, travel well, and should last forever.
The only difference is the magnet in the drivers as far as sound is concerned, but more importantly the 7506 has a 'pro' very long coiled cable while the V6 has a shorter, straight cable.
From what I understand, the V600 is not in the same league, but you should check that at Head-Fi.
Whoops, I guess I was just wrong about that. It was the only reason I bought the 7506 over the V6, but I guess I was misinformed somewhere along the line.
Besides brand there are some other things that one should look at like the cost and also whether headphones offer a natural means of listening. Remember ther is part of your health that you want to check.
Wikipedia on the MDR-V6, MDR-V600, MDR-7506, MDR-7509HD, and MDR-CD900ST: http://en.wikipedia.org/wiki/Sony_MDR-V6