It seems to me you're not likely to get the best deal on your "trade in," just like you wouldn't with a car dealer. Personally I'm pretty loyal about stuff like this, and if both shops sell the same brand then I don't think I could bring myself to shop at the one that's not *my* shop. But that's just my personal sentiment and not an ethical argument! I think others are right about doing what's best for you in this situation.