Good RGB monitors would have higher resolution (finer mask), higher quality, faster electronics for better signal fidelity and higher bandwidth (for higher resolution) and (depending on intended application) phosphors with longer persistence to avoid flicker.
That wasn't really a thing back then. The Amstrad CPC home computers had a tube with a finer shadowmask than the "proper" PC monitors of the day, after all.
Domestic TV shadowmasks were way way way finer than the 320x240ish resolutions common at the time, and indeed could be finer than 640-pixel horizontal resolution. Bear in mind that a broadcast-spec camera at that time ought to resolve 800 horizontal lines.
