I've got two old steel bikes. The lighter one has 25's, the heavier one which I've been using for commuting has 28's. When I ride the good bike I pump the tires to their max, 110psi. I pump the commuter to their max 105psi every so often but they drift down until I pump them up again a few days or a week later.

On the commuter I can feel the difference in how smooth the ride is when I pump them up. I think they also have less rolling resistance when I pump them up. The 25's on the lighter bike ride harder and seem to roll more easily. Part of the ride difference may be the geometry of the frame, not the tires; it has longer chainstays and I'm sure shallower angles.

But the difference between the 25's and 28's when they are pumped up is pretty subtle. If you rode blindfolded you probably could not tell the difference (but you might run into something).