There were several early-adopters of the Andersen on here a while back that posted a lot of data. The general consensus, IIRC, was that once the trailer gets north of the 5000lb mark, it had a difficult time transferring enough weight off the tongue to balance the weight distribution adequately.
^^This is the most common complaint I have seen. I would stick with the Equil-i-zer as it is proven to work with that size tt and you already have it. Just make sure you have adequate bars. There are pads you can get for them to reduce noise.
Sounds like the dealer is just trying to make a buck on something you don't need.