RVM is larger and does a whole lot more than chruby. For one, it's much more friendly to new users and doesn't suffer from usability problems like these:
RVM doesn't suffer from those specific usability problems, but it most certainly does suffer from usability problems. I've wasted many, many hours debugging issues caused by RVM. For the most part, those issues are because RVM is larger and does a whole lot more than chruby, and since switching I haven't once missed any of those extra features.
https://github.com/postmodern/chruby/issues/212
https://github.com/postmodern/chruby/issues/213