I've had both (305/40 now) ... barely any noticeable difference sidewall wise, the ride is slightly stiffer on the 305s.
I meant compared to 275/55/20. Not sure if loosing 1" of sidewall is going to be a wise decision with the technique my wife sometimes uses to find out if she's parked close enough to the curb. The extra width of the 305/40 may help protect that lip though, and look much better than the 285 too.