28 may improve the aero drag over 32 (from Roger Musson) but the weight saving is marginal.
32 looks to be the standard, and there is no real reason to go against it.
I did 32 because for me they look stronger with more spokes.
My brother got his wheels built from Peter, and are great too.
Unfortunately he was hit by the random 48.5% anti-dumping tax for built wheels( it seems particularly enforced in Belgium, Spain and Italy ).