Well 990s are open back, so that decision should be obvious - get them if you like open back cans and don’t need to isolate yourself from room noise, don’t get them if… well none of that applies. 
As far as the 32 ohm and 80 ohm, they sound very nearly identical, especially after you listen to either of them for more than a few minutes in a row. The only difference that’s practical between the two is that the 80 ohm comes with the plushy earpads (which I bought for the 32 ohm which comes with hot sticky leather pads that I hated) and the 32 ohm comes with a 1.5 metre cord which is WAY more practical for things like plugging into a laptop or a phone, but much less useful in a studio environment where you want to move around and not be tethered to the interface. For a handy decision chart:
- Do you care about how long the cord is? Short: 32 ohm 770. Long: either of the other two (990 is coiled, 770 is straight, both are ~3.3m when stretched, IIRC)
- Do you prefer the sound and space of open backed cans (990) or do you like the isolation and pristine solitude of closed back (770)?
If you end up getting the 32 ohm 770s I highly recommend buying the plush earpad replacements and putting them on day 1. But other than the short cord, I see absolutely no reason to care about the impedance for any of these. If you have a good headphone amp, the 80’s will sound as good as the 250s for anybody but nitpicky audiophiles who only listen and don’t bother creating. No cans are perfect, and these come as close to the ideal as any other reasonably priced high end ones.
A final note about the 880’s: they’re just like the 990’s but a little brighter with more sizzle. I personally find the 990’s compare well with mix translations to nearly any stereo or playback system, so I feel confident using them for mixing, but if your ears prefer the 880’s, then drop them in place of the 990’s. To me, the 880’s were ever so slightly fatiguing. I haven’t had that issue with the other two models. The 770s have a slightly more “pop” sound than the 990s but I’ve used them to mix and track when I don’t want to bother my studio mates and you adapt pretty quickly to the subtle differences and so I wouldn’t worry one whit about whether one is ‘better’ - choose them on the other priorities I mentioned and you’ll find a pair that will, after a little bit of listening, become just right for you.