In a relatively-close audition, I'll go with, of two actors who both read well, but one has the "look" better, the one who looks closer to what I want.  And the main reason for that is, I think most actors realize that appearance counts for something, so it's "fair game" to take that into consideration.  Whenever I go into an audition (as an actor), I always try to tailor my appearance to the main role I'm planning on selling myself as, eg, do I wear business casual or a muscle shirt and jeans?  Do I comb my hair or leave it messy?  Do I shave or have stubble?  Etc.  Simple changes that I try to consider a few days before my audition so that I don't come in "looking all wrong."  And if I feel like there's some big bar to that, I'll tell the person I'm auditioning for (eg, if my hair's a certain way for a show I'm in and it should be longer or shorter or something, I'll mention to the director that I'm willing to make the change).

If, however, there are other factors weighing on your decision, eg, one of the actors you've worked with before and you know they're either really responsible or irresponsible, etc., then go with those.  Every once in a while, I'm torn between someone who looks the part, is plenty talented, but is an unknown to me vs. someone just as talented who I've had positive working experience with in the past, but doesn't look quite right.  If I go with the person I know better, I'll have to deal with the disconnect in appearance, and I'll often find some creative way of doing that.