I don't have strong views about the G250 - I've used them and they worked as they should - but the Mk25 is the only first stage I've used that has gratuitously freeflowed. I've had both Mares and Poseidon HP seats fail, after much use and no service for three years, but both I and a friend who still swears by the Mk25 have had cold water freeflows, in water that really wasn't that cold (Stoney Cove in England in September, about 15C). Not just one regulator that might have been maladjusted, but several. I used to own 9 Mk25s for rental in my dive centre, but I phased them out for this reason. My favourite warm water first stage now for rental purposes is the Aqualung Titan. Remarkably high performance, very consistent easy breathing, and requiring minimal service attention. And cheap. In rental use I had to tweak the Mk25s several times in a year.