By and large, weight.....
Except for certain high volume shooting sports like Dove and Brit style driven Pheasant, most field guns get carried much more than shot. Clay game guns, t'other way around.
IMO, field guns for upland stuff should run 7 lbs or less. My trap gun runs a bit more than 8, and that's light for a trap gun. Skeet guns run to long bbls these days, and except for the SXS fans, tend to run close to 7 3/4 lbs up.
Both skeet and field guns should shoot flat,and are stocked(Theoretically) to do so. Trap guns shoot high.