Hey, Being a bike salesman sometimes, I would say he was not lieing, but that the differences is not always noticable to a less sensitive rider. Go test ride both sets of components and see what you think, because that's whose opinion matters in this aspect of componentry. I will agree with some of the other posters and say that weight and durability change from level to level in shimano, with increased durability untill you start sacrificing too much to the god of weight-weenies (which is really only an issue with a few pro-level components meant to be used for racing and not training). Generally in Shimano's mountain line you want to at least buy at the deore level and in their road line I would say a Tiagra/105 mix bike would be as low as you should go IF you are someone who puts above 70/80 miles a week on your bike and sometimes rides in crappy weather. If this is for a 5 mile ride or two a week go as cheap as you like it.

Anyway, test ride lots to help you decide what you personally want to pay for.

Oh, and Sora levers suck for all but the biggest hands. You can't shift one direction from the drops. Blech.