Even though Ruby on Rails seems to be the most popular with the most job openings, is there a compelling employability reason to learn django for example? Maybe because everybody's learning RoR and there's a shortage of django developers in relation to the demand? Or is php along with a popular php framework the way to go simply because of the sheer volume of php related jobs out there?
It depends on many factors
- Your location
- Firms that you want to work for
- Kind of web application these firms are building
- And even all of these wont point you to that one holy grail.
Learn what you like most and think you can accomplish your tasks with.
You'll have to look at the job listings in your local area, everywhere is different. And I would say that if Ruby on Rails is "the most popular with the most job openings" where you are, then your location is more different than most!