There is some degree of truth that more HP doesn't change the speed that much.
Pontoons are not speedboats and they never really "plane". You do get a degree
of laminar flow across the pontoons and there is definately a step but it takes
a lot more power to get a tad bit more speed.
When I went down from 75 to 60 my top speed changed about 1-2 MPH.

Ask them what the prop recomendation is for both and what the predicted RPM is
with that prop.