The DS series has 700x38c tires and the FX series (7.3) has 700x32c tires (6mm difference). Besides the tires being wider on the DS, the tread pattern is what makes the biggest difference. The FX series has a much less aggressive tread pattern then the DS series. This is mainly do to the environment that they are designed to interact with. The tires on the DS series are designed to work well in gravel and dirt, while the FX series tires are designed to be used on smoother surfaces, such as pavement and hard packed gravel.

As far as the weight is concerned, I couldn't find anywhere on Trek's site that lists the weight of each bike, but I would be willing to bet that the DS series bike is much heavier than the FX series. I have ridden both of these bikes (last summer) and there is a considerable difference between them. My advice would be to ride as many different bikes as you can before you actually purchase one. That way you will find the right bike for YOU. Not everyone will have the same opinion on any given bike.