I have single-handed a 12.8 for short periods during a watch. The original 12.8 cutters all came with easily stowable jibstays and electric winches, and most have high quality autoppilots, which made them a breeze to sail short handed.
The 12.8's are quite weatherly, pointing quite high (especially with thier centerboards down). The Whitby 42's and Brewer 42's (depending on their configuration) do not point all that well.
The 12.8's do not appear to have any bad manners under power and I have been vvery impressed with my father's ability to handle his in tight conditions. The Whitby 42's would be harder to handle in tight conditions since they are less manueverable.