"1) will the 18 gauge Karran sink feel noticeably cheaper/weaker than the 16 gauge Kraus sink we are considering?"

No. I've installed plenty of Karran integrated stainless steel sinks and have owned one. Great sinks.

" And 2) has anyone had the interior edges of the sink “built up” to have a thicker appearance, similar to what it would look like with quartz or granite?"

This retrofitted Kohler 3943 has had the sink edges build down to the same thickness as the front edge. It's a bit more work for the fabricator, so expect to pay for the look you want please. This has a slight positive reveal which never catches crud and protects the bottom of the top from chipping.

This is the Karran 540. It has the best possible edge detail. No maintainence ever and no crud hiding lip as with the negative reveal and no elongated fraying contaminated unsanitary silicone joint as with the flush:

"Two questions: 1) will the 18 gauge Karran sink feel noticeably cheaper/weaker than the 16 gauge Kraus sink we are considering?"

No. I've retrofitted plenty of Karrans and have owned one.

" And 2) has anyone had the interior edges of the sink “built up” to have a thicker appearance, similar to what it would look like with quartz or granite?"

Yes, but it's a bit more work for your fabricator, so expect to pay for the look you want please.

Karran 540. That's the best possible edge a sink can have.

This Kohler 3943 has had the sink edge build down to the same thickness as the front edge. It has a slight positive reveal, the second best possible. No, it never catches crud (the negative reveal hides it) and protects the bottom of the top from chipping. The silicone joint is maintenance free and never elongates, frays, and becomes black with unsanitary contamination as does the flush:

Thank you so much for taking the time to post these photos and explanations— so helpful for us!