The problem is we're just around the corner from faster monitors of both. 4K maxes out at 60Hz currently, which is a waste IMO. Why limit yourself? That said, many AAA titles will struggle to get those kind of FPS rates anyway, but still, to spend over £1K on GPU's and then hogtie them with a 60Hz monitor seems a bit mad to me, so if you had your heart set on 4K I would wait for faster monitors to come along. Once you experience the smoothness of 100Hz+, 60hz really does feel a bit ancient.
In the meantime, I would go with 21:9 without doubt. Same situation applies though, in that faster monitors will be along later this year, breaking the current 100hz barrier that the X34/PG348Q manage, but overall 21:9 provides a MUCH smoother and more immersive experience, easier to drive, and even with a single 1080 you'll be fine, especially with the inclusion of G-Sync (which is ESSENTIAL).
This would be my recommendation, and you won't regret it. 21:9 is the future in my view, 16:9 just doesn't cut it any more once you experience that wider aspect ratio for any length of time. If you get a single 1080 and 21:9 monitor now, see how you get on and when faster 4K monitors arrive the 1080Ti will likely be out and you can consider switching up then. But here and now, 1080 + 21:9 100Hz all day long.