I think that sinks and faucets in particular, and to a certain extent even towel bars and other accessories are either more round or more angular. I'd say look at some of your other fixture choices and see which one they seem more like to you, then consider repeating that to gain continuity throughout the room.

For instance, we're going with the Kohler Margaux faucet and the Kohler Archer sink. I look at both of these as being more rectangular than round, so we're going with a rectangular mirror. Does that make since to you?

Perhaps you could cut out pieces of paper in the sizes and shapes of the mirrors you're considering and hang them up as they would be in your bathroom and see if one feels better than the other.

I like frameless mirrors for the bathroom, especially if you have a lot of other ornamentation and trimming in the other features. I don't know if this counts as frameless, but I like how the mirror works when there is one wide swath of mirror all the way across the vanity from edge to edge, top all the way down to the backsplash. I just like that I can see my whole body (or most of it) in the mirror if I am dressing instead of just my head and upper body.