Generally what you will find is less, often much less, barrel and/or complex distortion from RF wides vs SLR wides since RF wides are much more symmetrical in design. This may or may not be an important consideration depending on what you photograph. RF wides also tend to be sharper/better corrected at the edges and corners, particularly at wider apertures. They are just much simpler to design and build than equivalent SLR lenses. What SLRs always have going for them, especially with wides, is you see more accurately.
Originally Posted by puketronic
To OP, like some others have mentioned, I'd suggest a 24mm might be a good option. This is one of my favourite focal lengths in 35mm. It straddles wide and super-wide quite effectively so it is fairly practical for many applications, whereas lenses 21mm and wider have that super-wide "look".