Berthoud is A LOT firmer than a Brooks in my experience. The Aspin also feels a bit narrower than the Brooks I had before.
The build quality on the Gilles is second to none as far as I'm aware.
Have you looked at Selle Anatomica ? I got a newfangled plastic one a while back and I think its very comfortable (Much better than B17s I've ridden in the past) also I think it would suit the aesthetic of this build.
Agree on the silver Campag, and I vote for brown. Maybe a nice Gilles Berthoud saddle if you're feeling fancy.