it doesn't hurt to see what company b has to say. just be honest with them and yourself.
you don't have to commit to company b, if it comes up just let them know you have another prospect and would like to see what they have to offer.
let your credentials speak for you ... if company b wants you, they will wait. that is if you are interested in their offer.
after all you are talking about a good portion of your life to pursue, and you must do what you feel is right for you.