Long-term, I lightly recommend Homebrew, which via its
brew cask command can install applications including both R and RStudio. In Terminal, install Homebrew (copy the command from the link above), then enter
brew cask install r-app
brew cask install rstudio
The benefit of this approach is that it makes it really easy to update later:
brew cask upgrade
Homebrew Cask can also install other full applications, which is sometimes handy, though Homebrew itself was originally intended (and is still heavily used for) installing and updating command line utilities, e.g. git, compilers, pandoc (for RMarkdown), imagemagick, hugo (for blogdown), etc.
To be clear, Homebrew is by no means necessary (particularly if the command line is still daunting), and you can always start using it later if you choose. It's pretty awesome, though.