At time of writing, on the Ruby on Rails download page, they recommend using Ruby 2.0.0 with Rails.
Ruby 2.1.0 was released at the end of December 2013.
Is that recommendation out of date? Do you have any tales of woe or joy about running Rails 4.0 on Ruby 2.1.0?
The current Ruby 2.1 version has some segfault issues with Rails 4.0. Several fixes has already been merged into master thanks to the work of Amman who upgraded GitHub a few weeks ago.
I would suggest to hold off the switch of Ruby 2.1 for now, until Rails 4.1 and the first bugfix version of Ruby 2.1 are both released.