If the comment seems to be of genuine interest, I'll answer. I do the same when dealing with prejudice with the Linux platform. What I don't do is give my time to the defense of something I like, enjoy and prefer to use. I have better things to do with my time, and I've found that these remarks are often made to see how much of a rise they can get. It's one thing to have a preference for what we like, it's quite another when we start arguing with others over which is better.
Respect is earned.
It isn't something you can receive by way of apology or by preaching that your OS is better than any other. Your goal should be to let others know that you use Linux, not to convince them to install Linux at their first opportunity. People who are only casually interested will be turned off with a hard sell approach. Instead, plant a seed of interest and wait to see what develops. My opportunities to introduce others to Linux generally come in one of two different ways.