weight, and climate and "float" are all factors. SPD pedals are notorious for being very poor at shedding mud but you can adjust the spring tension for how hard it is to release or not. Times are supposed to be very good at shedding mud. think Frogs have the best float which is important if you have knee issues.

If your local shop is being condescending either take your business elsewhere or have a little talk with the manager.

Irulan