I don't have experience with the bibs but for anyone to comment accurately they will want to know what kind of riding you will be doing, duration, temperature, etc.
The AmFib does not have a chamois, I think, check me on that. So the advantage would be cheaper price and you could wear them multiple times without washing because you would have cycling shorts underneath versus having to wash the Gavia more often. That and the additional insulation you have from wearing bibs and shorts versus just bibs. I have chamoisless tights but often wonder if it would be more comfortable to not have to wear two layers. I don't seem to get cold down there, I generally need less clothes overall than anybody else, so I am thinking about tights with a chamois. Since I ride multiple times per week it is nice not to have to wash them all the time. I hope that helps.