What can one expect from coupling a 16:10 signal to a 16:9 monitor?
You're right in theory that a 1920x1080 resolution will map perfectly onto a UHD (3840x2160) display, and you're also right that it will look worse than 1920x1200.

However, most monitors will perform some sort of an interpolation (smoothing) for all resolutions other than native (source).

While "how ugly will the interpolation be?" is subjective, I can however say that in my experience the interpolation doesn't drastically affect picture quality.  For instance, I have a Retina 5k (5120x2880) iMac, and I've compared UHD with WQHD (2560x1440) on it, and the UHD does indeed look better (though obviously not as good as 5k).

So if you do get a UHD monitor, I would say the 2560x1440 resolution would look the best. The 2560x1600 mode will suffer from an anisotropic scaling problem -- images will look squashed vertically (see this link).