There is a difference between a fork with a carbon steerer or an aluminum. It may or may not be noticeable in the ride to many people, but there is one. There are advantages and disadvantages to both. If you have an aluminum steerer tube the stack height (the amount of space between the top of the head tube and the stem) can be greater. Many manufacturers of all carbon forks don't recommend going above 40mm. However, given the same design and materials, the aluminum one might be a bit heavier. So, the carbon one is likely to be a bit lighter. In any event, either frame and fork should serve you well. I would guess that the dealer might be paying a bit less for the ZW6. So, I might ask for a discount. I don't think I'd settle for anything less than the labor for switching out parts as free. Hence, I would think you should be getting this bike for a bit less, or receive some other compensation.