Favourites. A lot of the time comments consist of just "nice" or some kind of emoticon with no real words at all. Not very helpful. Favourites are a nice way of seeing how much people actually enjoy my art.
Hard to say, Comments are more "exiting" to get. But I must admit that any more than a dozen at a time can get tiresome to answer to. Most days I log in to about 6 or so so its manageable. But sometimes I've truck it lucky and got a hundred or mor eovernight when I something something or get a DD. It's flattering, but after I'm done I'm so glad to see an empty message center (it's not that I get bored of comments, my fingers just get tired from typing so many responses )
Favourites I feel I can ignore without coming off as a douche I usually thank those who fave more than one artwork though, but the rest I don't worry about
Even when it's a one-liner though, I still appreciate the fact that someone took a moment to say something. I myself don't always have anything to say, so when someone does put in that effort I appreciate it.