Well, it seems like the hotter the engine is, the harder the radiator hose is, which seems like more pressure. I would think the 16lb cap would release and start filling the overflow bottle sooner than the 25 lb.A radiator system with a 16lb cap will NOT cause or allow anymore water to go to the reservoir then a 26lb cap. That is dependent on the system temperature.
Water is basically in-compressible at those pressures. It will expand exactly the same percentage based on temperature at both pressures.