Emacs + ESS? Way more flexible. Maybe less integration because many of the big R package devs work for Posit. RStudio has a lot of superfluous junk in the UI I just don't need or care about.
I've used ESS for the past few years and recently tried using RStudio when I'm on Windows. For my purposes, which is just a little industrial statistics on the side, they are remarkably similar. I feel right at home in either!