I recommend taking the course you know the least about. I've found that doing new things early is a great way to make progress. It gives you insight into talents (or distastes) that you don't know you have. It exposes you to new ideas that you can mix with knowledge you have, and enrich that knowledge. It gives you a broader knowledge base for the next thing you learn. Do you know a biomed engineer whose work you are interested in? Ask that person for advice, too. My personal choice? I'd go for biomaterials, because I know nothing about them.