Gereke, you are completely right. The salesperson should have told everything he knew about each vehicle. The LST2 and MGT included. He could have been told to push Traxxas, who cares? Doing the right thing is better. As far as I know, and a little personal experience, Traxxas vehicles are less reliable, the other MT would have the buyer excited and coming to spend more money. An impulse buy can be worse in the long run because he made the wrong decision. He might end up getting out of the hobby completely.
I don't think I would've interfered but depending on what the salesperson was actually saying, I might have. I've done it before at my LHS. They are good guys but they all own old Traxxas vehicles and primarily try to get money out of you.