Having flown both the Wessex and Seaking in the SAR role I can see nothing wrong with the decision to have a mixed fleet. During my time I never had a situation where I could not carry all the survivors I was presented with and can only think of one where the Seaking was fully laden and that was off Ireland.
There is a need, however, to make sure that the mix is based evenly around the country and in a way that it is mutually supporting. The recent incident had both the HMCG S61N and Chivenor's Seaking in attendance.
Don't forget is wasn't that long ago when the UK SAR fleet was a mix of Seakings and Whirlwinds and there weren't any cases of lack of lifting capacity for over water rescues that I can recall. The 139 is a quantum leap above the Whirlwind in both carrying and all weather ability
HF