GUI choices?

Two 2560 x 1440 27" monitors... or one 3440 x 1440 34" monitor? What is the best setup with separate video monitoring over a Decklink?

With separate monitoring I consider the ultrawide a nice option.

Well the two gives you a lot more pixels so that would be my vote. Just stay away from UHD.

My choice would be for a single 34", 3840 x 2160 monitor.

So many choices, so many opinions

I think you may need to try what suits you.

I'm a big fan of just a single 27" 2550x1440 display, but my co-colorist/partner prefers two 27" GUI displays. I also prefer a third display dedicated just to external scopes (which is actually not that expensive if you can find an inexpensive MacMini and ScopeBox).

I'm not a fan of the UltraWide displays for GUI for the simple reason that I have to swing my head too far to the right to see the keyframe display, which I use constantly. I totally get this is a subjective call and there's no one "right" way for multiple people to work.

In the old post facility days where there'd be a dozen color rooms and two dozen different colorists having to compromise, there were angry disputes as to which side to put the Vectorscope, which for the Waveform monitor (both separate displays), whether the GUI should go on the left or right, and so on. There was also a time when we had a big HD display and a smaller one for SD (in the 1990s)... and in ancient times, we had one monitor for PAL and one for NTSC, using just one per session. All very much individual choices.