One is blue, one is black. The blue one is the older model, and is more rare (I think the seller is just taking advantage of that). I don't think there is any difference between the two other than color. They're both foldable.
They all have decent isolation, but the D2000's have the least of the group. I notice a minor difference between my D7000's and my Pro 750's, but nothing major. That loss in isolation is because of the housing's construction. The construction gives them a better soundstage. Soundstage is the size of the hypothetical room you're listening in. The 780's will sound like the sound is coming from the inside of your head, the Pro 750's will sound like there are speakers close to your ear, and the D2000's will sound like you are in a small room. A larger soundstage is preferable, but I don't know how important that is to you (and honestly, you won't know either until you experience both).
The Pro 750's, HFI-780's, and D2000's have great bass. The D2000's are the most comfortable of the group, next comfortable are the Pro 750's.
The D2000's will work the best out of the three from an iPod. They require less energy than the other two (not by a huge amount, though). I would still recommend an amp and dac for any of these if you want to get the most from them. I'd recommend something like a Fiio E7/E9. They're both a DAC and Amp combo, and they're great for the price. Only difference is that the E7 is portable, and the E9 isn't. The E9 also can drive more demanding headphones (none of the ones I've recommended are demanding).
Thing is, you can get the DAC/Amp later. It's what I usually do. The Fiio ones aren't too expensive, the E7 shouldn't run you more than $100.
They are all modable to some extent, the D2000's are by far the more modded headphones. After that come the 780's. The 750's aren't often modded, but the Kees mod is super easy to do, and if you don't like it, you can just unmod them in a minute.
You can think of modding as "upgrading" the headphones with a nicer cable, some sound-dampening material, new ear pads, or new cups. You may be into that sort of thing, you may not.
Also, Ultrasones usually are love-or-hate headphones. They have a very controversial house sound that will either make you think they sound fantastic, or think they sound awful. Looking at the music you listen to, you will most likely be in the former camp. They're great for electronic music, but pretty bad for acoustic or classic rock. The house sound of the Denons are similar, but they have more of a midrange (making them work decently with acoustic as well as electronic), and are warmer. The 750's are cold and precise, intensely fast and surgical. They're both good in their own ways, and both will serve you well. It just depends on which one you want. If you have the cash, you could buy both, and use them each for about a week (it takes a few days to get used to a pair of headphones, before that they won't sound as good), and return the ones you don't like. It's a common practice around here, but you *need* to make sure the online store has a good return policy before doing this.
Edited by Taowolf51 - 1/3/12 at 4:50pm