Favorites are like a nod but not visible beyond the conversation. Boosts make the post available to every one in your timeline. So if you think it’s fun, funny or worth other peoples time you should boost. I usually reserve favoriting for comments on posts I’ve already boosted.

I’d be more concerned about the older cat passing something to the younger cat. But if the older one is up to date on shots, indoor only and not FIV positive, the risk is minimal.