I'll be honest...
If this argument was about trying to influence someone on what their future purchase should be, I'd join in and probe a little bit and find out more about the person who's making the purchase. If I felt this is a person who would benefit from Apple products, I would then move onto explain how the rest of the Apple ecosystem works and how it manages to tie in all products so seamlessly. For me, that is the biggest appeal with it comes to owning Apple stuff. Everything "just works" together so easily, so I would sell them on that.
Now, if the argument is just a matter of two nerds battling over who's got the superior product, then, no, I would stay out of it because there's nothing more frustrating than arguing your points with someone who's determined to "win" and be right no matter what. I've had those arguments before (iOS vs. Android) and I've come to realize that it can be as messy as debating religion, politics, or abortion.