the 1:2,0/28 is a rather old design and you cannot expect more from such this wide opened lens fully open. A 1:2,8/35 however is a well proven design which is much easier to correct. Take the 1:2,8/28 AiS with floating elements (not Ai!) this is one of the best 28 mm lenses on the market.
The 28/3.5 dates from 1959, it was one of the original lineup for the F.