Mild needs just enough carbonation to impart a bit of mouthfeel and to drive the aroma out of the glass and up to your nose. Too much carbonation and the beer becomes dry, harsh and acidic. Gentle carbonation can make the beer feel creamy. Target a carbonation level of 2 volumes for bottled, 1.5 volumes for kegged and just over 1 volume of CO2 for cask conditioned beer.
If all vessels were carbonated the the same CO2 level, would there be a difference between the percieved carbonation level when the beer comes out of a Keg vs Bottle vs cash?