This is a weird discussion to me.
You have an 18x120" South Bend lathe that works.
The Atlas 618 is a 6x18" lathe
The Shop Fox M1099 is a 10x26" lathe
The Grizzly G0791 is a 12x36" lathe / mill combo
You like the Atlas, I like these little lathes too so may have a bit of bias. The flat ways and cast zamak construction, is much less of a concern on such a small lathe. They are small, handy and take up very little space. You like it so I don't see much reason to get rid of it unless you find it just not getting used or is getting in your way. You have a big lathe for projects that are too much for a 6" lathe.
The other two... the G0791 seems the more useful, being a 12x36 with a full QCGB. The mill may not be great (if you get it working), but it doesn't hinder the lathe, doesn't take up much more room and you never know, you may even find times having it is useful.
The Shop Fox is probably fine as a small lathe, but of the three seems to be the most redundant. It is not as small and handy as the Atlas or as capable as the Grizzly. Probably not a whole lot smaller than the Grizzly either.
Or keep all four, use them and see which ones you keep coming back to.