I have the navy/black in the aged calfskin (which would be last year's Gabrielle; I don't think they have done lambskin), and the ombre gold/black from pre-fall this year, which is goatskin. The goatskin is definitely stiffer, but long term, I think it will crease....maybe not as much as the aged calf, but it doesn't feel stiff like a flap or more structured bag. It's thicker, but overall, I think the goatskin will still slouch, if that makes sense. I personally don't mind the slouchy look of the aged calf; it's super soft, and it's held up very well. Are you able to go in and try both on? They're different vibes, but it's a great bag that's also practical. Before I found my new navy/black Gabi, I ordered a couple of different sizes from
Fashionphile to try on and see the leather/chains (30 days free returns!). If you decide on the aged calfskin, I'd look at pre-loved first. Had I wanted all black, I found 2 great Gabis that were in perfect condition at nearly $1k less than new. I'm assuming you know that the Gabi is slated for a price increase in a few days as well? Good luck deciding!
P.S. Pictures of both my aged calf and goatskin Gabis are on pg 74, posts #1108 and #1109 of this thread, if that helps!