My sister, a former Tophat Mk 1 owner and now owner of a lovely Valiant 40, tells me the earlier Baker built Tophats are the most sought after as being better built.
The Mk1 and 11 S&S34s involve major changes to the keel (hydrodynamic shape), rudder (deeper spade rudder on the Mk11 versus skeg hung on the Mk1) and taller two spreader rig. I have sailed both and the Mk11 is better downwind without doubt. The Mark 11 rig was modelled on Morning Cloud, Ted Heath's 1969 Sydney to Hobart winner.
Some Mk11s have the engine in the centre above the keel but as time moved on later boats and those that have been reengined have often changed to a saildrive under the cockpit, in both Mk1 and Mk11. Many Mk1s now have the taller two spreader rig e.g. Morning Gold. There are very few Mk11s. My boat is the last original Swarbrick built S&S34 (launched early 1984) and it is a Mk1. The Mk11 shape cost a few thousand dollars more and I think some buyers wanted the stronger skeg hung rudder.
Big changes often happen to models on all yachts without a Mk change. For example, the earlier S&S34s are polyester resin and the later (roughly when they changed to a square fore hatch) are epoxy resin. The epoxy resin is supposed to be stronger and more osmosis resistant.
I'm not sure the Mk is the most relevant consideration, look for the features on the boat.
