Knowing more means he knows how to do a better job, and that is
definately worth $700.00!! I don't understand what the damper controls
from your post, but I find very few contractors who understand air
flow. That is a BIG plus. Go with #1. You will probably get more
than the damper and radius elbow for the $700, but they may all be
little things that add up to a lot over the years in better comfort and
performance.

Stretch