I haven't heard Sennheiser, so I can't really tell you which is better.
But I don't think you can go wrong with D2000.
I bought Denon AH-D2000 yesterday, and I listened to it for a good 10 hours. I am very pleased with it. Depending on songs, there are times when the the sound gets a little 'messy' and a little harsh on high frequency, but nothing a burn-in can't fix.
Just for the record, I am using Claro Halo sound card as my amp & source.
There are a lot of threads discussing those two cans, and I sugguest you read them too
Bad comparison, but i've had a great time when I decided to listen to rap through my Denon's. Just yesterday I tried Snoop Dogg and the Eastsiderz, which has a lot of programed bass for sub-woofer, WOW that's was something else.
as others have said, these two aren't in the same league. The D2000 should be miles ahead of the HD212.
Ra978R: he has another thread where he makes weird comparisons as well.
Price is a good -general- indicator of sound quality. -generally-, the more expensive a set of headphones is, the better the quality. Therefore, when comparing two headphones of decent brands, you can roughly assume a headphone that costs 6x as much is going to be way better.
There are, of course, exceptions to this rule.
Some brands, skull candy, Bose, Beats by Dre... may be overpriced however.
Some cans show tremendous value:
Koss w/ KSC75 or KSC35 drivers.
Various cans by JVC