I don't think there's much comparison between the Goat Island Skiff and the Core Sound 15, besides similar length. I think a more fair comparison would be between the Bay River Skiff and the Goat.
The CS 15 is so much more boat than the GIS-- side decks are one huge difference, for starters.
The CS 15 has a deeper entry, a bow that doesn't hang up on waves, and deadrise to give her an edge to run on while level.
Most basically, the CS 15 feels stable with four crew members, while the Goat feels stable with up to two experienced crew.
It's a fairer comparison between the Bay River Skiff and the Goat, but the BRS is way more stable-- in that it affords one the opportunity to focus on things other than where to sit and which way to lean.