Power valves are always a concern unless the carb doesn't have them.
Metering rods and vacuum pistons are a concern on Edelbrock, Carter, Rochester, etc.
The gas mileage will probably be slightly better with the Holley because the primary is smaller - but this assumes both are adjusted equally well. Out of the box, who knows?
I'm slightly more comfortable with Holley, and that's what I would base a choice on - are you familiar with or have spares for either one?
