One company with an impeccable rating and a long history in the area proposes to collapse the existing windows' aluminum frames and use the existing opening. (Sorry, I am probably missing the correct terminology here.) To keep the [somewhat thicker] vinyl windows from jutting out unnaturally, they propose to cut into the sill/trim somewhat so the inner surface of the window will be an inch or so closer to someone standing inside, compared to the existing windows. I believe this would not involve the addition of any trim outside.

Another company, also with excellent ratings, would use the same technique only on the living-room bay window. Everywhere else, they would make a cut into the siding about 1 1/2" away from the existing frame, remove the old window as a whole unit, add a metal flashing strip at the top, nail in the replacement, and add 2x2 trim between the window and saw line.

For the experts here, which technique do you feel is best? I am sure that if I asked the company reps, each would have something derogatory to say about the competing approach.

Also, it seems that I am being driven toward windows by Comfort Design (formerly CDI, a small company out of Tacoma), by their willingness to substitute LoE-180 glass in place of the usual low-SHGC stuff that everyone else gets by default. Specifically, both contractors want to use CD's "architectural series." The claim is that this is a decent-quality line, superior to e.g. Milgard Style line. I've seen queries here before, but no informed opinions on how good the Comfort Design products are. Thoughts?