I would be careful with Rails, when you plan to do some heavy data processing and/or if you need your application processes to run for a long time without being restarted.
Heavy data processing yes, but the long running process is bogus. The major memory leaks in Rails were patched up years ago. The tooling is a lot better today on the off chance that you do need to debug a memory leak, and the new breed of ruby runtimes such as Rubinius will keep memory in even tighter control.