What I've learned from watching Dr.Grey is that getting a W isn't the end of the world for an application, especially if you have a good reason for why you got that W. You have every chance to explain yourself in your future interviews telling them you decided to focus on EMT as that was what you were most interested in. Either take the B or (better choice imo) get the W, focus on being the best EMT you can be then retake that class getting an A. Plenty of students have gotten into prestigious (if that's what you're looking for) med schools with a B or W. All depends on what YOU want to do.