Mounting really isn't much of an issue as long as you have access to the bottom of the fuse to add a rear mounting location
Between those two, I'd lean towards the Stinson over the Decathlon, I've had bad experiences with float plane with full symmetrical airfoils and high wingloading adding floats creates
I Built a Great Planes Decathlon back in the days,, it didn't end well,,, I added a float to a Extra 230,,, the was a fail also
good luck