Maybe I’m lazy but I feel like I’d have to turn my head too much for a 21:9. I do like a bit more vertical space than a 27" 16:9 too. I’ve used a 28" 16:10 before that was really good (size, at least, but only 1920 x 1200).
I think my next monitor would be a 4K 40" and I would sit a littler further back from it. That’s my plan anyway, if anyone makes a good one under £400 max.