Rstudio is a better editor for R. The problem is that it's a worse editor for everything else, and these days I write plenty of else, too. Being able to flip between different files, language shells, terminals, magit buffers, etc. effortlessly, makes putting up with worse autocomplete in R a pretty good deal for me. It does annoy me a little, but R is hardly a verbose language, and elisp is high-level enough that I have a shot at extending / changing the behavior if it really bothers me.