Inside mount to me is a more finished look. If you have nice trim on your windows it is a shame to hide them. I dont think there is any hard and fast rules, just a matter of personal preference.

I agree with what roseabby has written.

I also think that if there are fabric treatments going along with them, inside mount would probably look better, neater, since the edges wouldn't get caught by the drapery closing and opening and everything would look planned out and custom. Outside mount, to me, is more relaxed, more "off the shelf" Target (not that that's bad at all! I love Target!).

If you have the room, inside mount is my preference. Sadly, I don't have any room for an inside mount. It is both a matter of preference AND the physical nature of the window.