Just a few days ago, I actually directly compared the DT880, DT990 and the HE500 using metal genre tracks, with 2 of my co-workers. It was a 3 way split...
I choose the DT880 because I liked the speed and excitement it provided, and I felt the DT990 bass was not controlled enough for and was causing a bit of masking in the mids. My co-worker chose DT990 for the authority and impact it provided, he disliked the lightness of the DT880 and didn't like the HE500 either as it seemed less cohesive. My last co-worker chose the HE500, for the detail, soundstage and clarity, provided for a more enveloping and resolving sound.
All 3 are excellent choices for different reasons, and so (as unhelpful as this will sound) is very hard to recommend one over the other. I felt the DT990 performed similarly to the HE500, just perhaps a bit more forgiving and more thump in the bass. My personal favorite pair is the HE500, as it's great with classical and soundtrack music (which I love) and better then the other 2 at most other genres. But I'm probably biased since it's the most expensive headphone I own.
I think the safest bet would be the DT990, as it has a nice fun sound signature, but it gets a bit boring for me after a while. The DT880 is more risky, as quite a few people have disliked it after buying it, but it kind of grows on you after a while (at least it did for me), and imo is a better general use headphone in the long run. The HE500 is heavy and can be uncomfortable, but it's performance scales higher then any of the DTs I've heard so far and with the right setup, it has the potential of becoming an end-all level headphone (unless you're looking at 2k+ stuff). However, unless you can get used to the weight and feel of the HE500, I won't recommend it as a workhorse headphone (I use it only when I just enjoy music, not while surfing the net/playing games), but maybe my neck is just weak or something.
Also consider your budget, are you really willing to pay almost double for a small bump in performance? Money that could be used to get a nice DAC or amp to power your new headphone.
Hope that helps, good luck!