Ultra wides are a pain in the arse and are impossible to orient properly to your field of vision. Give them a wide berth right away.
Single vs dual, IMO, is a matter of space and habits. Dual will quite simply always win for people multitasking (even within a single app) like CG artists and engineers tend to do.
Dual 27" gets a bit too bulky though, and the dot pitch of a 27 at 1080p is kinda awkward (but that might be because I’m used to different).
Personally: Dual 24" 1080p or 1200, or 27" 1440p with one additional monitor of any description for browsing/reference/spare windows. If you can find a tilting 1440 x 900 monitor that’s not too big those actually go nicely alongside a 27", as you can orient them vertically and be pixel aligned to the other for very cheap.
I run a 27" 1440p and a 24" 1920*1200, I sort of regret having bought the 27", or I would if it wasn’t for the excellent colour, support for this resolution in DCC apps is plain bad, and the gains not that significant.

