because he didn't really know either. Usually in common American usage, rubber is the most general term and latex is slightly more more specific. Depending on context, I sometimes can tell if latex is refering to natural rubber (derived from the sap) or a synthesized product. Much of latex paint today is an acrylic emulsion with no latex ruber at all. (which is one reason why wood windows and doors painted with latex paints stick and jam - use oil/alkyds)
http://encyclopedia2.thefreedictionary.com/Latex+(rubber)So you see its going to be very hard to answer your question since the salesman didn't really give you enough information to base a decision on. They might diffrent material, but they might also be different constriction and even different profile. In fact, my experience is that you want to see the profile (cross section) of any so called reproduction weatherstrip part.
edit: a Chebby? There should be lots of sources, but if you have trouble finding cross sections or just getting the run arround on facts, try Restoration Specialty Supply (might be Restoration speciaties supplies, inc) in Wind gap or windber, Pa. (Sorry my catalog is not here). They may even have an on-line catalog now. Mopar stuff too. They made the belt weatherstrips (window felts) for my Baracuda using the correct material based on the originals from the car. The incorrect ones of course didn't function very well.