It's not completely stupid. My answer would be: If you need something up and running fast, but don't care about performance per instance, such as a marketing l

It's not completely stupid. My answer would be: If you need something up and running fast, but don't care about performance per instance, such as a marketing landing page use RoR. If you need something very performant per instance and are willing to pay for it, such as a enterprise application or large web site, use Java. If that's not an acceptable answer, the interviewer doesn't know what he's doing.