Jeez dude calm your hormones
Ok for the girl you're dating if you don't feel anything for her then there's no point in being with her. Might as well end it and not waste each others time.
As for the girl that dates Austin, if hes a good friend, I don't think its worth losing friendship with him for a girl that already said she just wants to be friends with you and nothing more. If they are both happy let them be.
So if I were you I would go with Samantha because you mutually like each other and has the best chance of working out.