To each his own, but the only reason to go with Ruby is in this case is if you already have a large legacy codebase and internal Ruby talent. For greenfield, stay away from both PHP and Ruby.
I've been using Rails for 15 years, and probably written or supported 20 apps with it. In all the reading I've done on it, and all the work I've done with it, I've never used JRuby, nor seen it implied that this was the "correct" way to run an app in production. As an appeal to authority, Heroku's default is MRI.
jruby was definitely the play in 2008-2012, reason being that puma wasn't out yet and everything else was doggedly slow, so you used to use tomcat or glassfish.
Since jruby is being interpreted into java (thus, bytecode), the resulting performance (throughput and latency) was a lot better and the lack of jemalloc (a good allocator by most means!) meant that your memory growth was a little more sawtooth and a little less... growy.
I think these days most folks are just running something like puma.
