He should be significantly cheaper, and he is younger. When I look at their career QBR, it's almost identical. Cousins probably has the stronger arm, but if he's not going to push the ball down field, then we might as well opt for Jimmy G. Garropolo's win percentage is substantially higher as well.
Honestly, I really wouldn't prefer either one in a vacuum. If I have to pay 30+ mil per year, as in what we may have to pay to acquire Cousins (or an additional high draft pick), I'd rather take my chances with Derek Carr, or just go cheap until something better comes along.