Bringing it up in the line before the event IS rude. Approaching him after the event and engaging in a conversation about what they liked and disliked would be the proper course of action. If they disliked something that system x does differently, then that would be the time to bring it up. Otherwise you're the gamer version of a religious pusher.
No it is not. It depends on what is said and how it is said and if the person kept it up even when the person he was talking to was uninterested. Otherwise it could simply be a discussion.
Several years ago I was at a game store waiting to see a demo on Dragon Dice. The person sitting next to me started talking about another dice he game he played and enjoyed and we had a nice discussion on the differences between the two.