You are comparing apples to oranges. If you use staple up you must extensively insulate from below. Perhaps you did and just failed to mention it. There seems to be some confusion about this whole subject. If you mount pex from below without a heat spreader you won't have good heat conductivity to the floor. That's just physics. However, you can get away with it if the ratio of R value below to R above is high enough.
Heat doesn't just disappear because you have insulation from layers of subfloor above the pex. It has to go some where. If one has no insulation below it will just go down because the ratio of R value above and below isn't good. It will twke the easiest escape route. But even without a heat spreader if you increase the ratio high enough of R value below to that above it will still work well. But it might work more like a concrete floor in that it will take a long time for that heat to work through the upper layer(s) of flooring.
I myself would go for pex mounting below with a heat spreader so I wouldn't have to wait so long. If you didn't heavily insulate below without a spreader then that was probably a mistake.
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