I've had a couple of the Aquia III's that you are considering for several months. The amount of pressure needed for the push button is very light, probably less or equal to what you need to push down a lever. Yep, having buttons on top mean you can't have other stuff there. On the plus side it makes for a very clean look and you don't end up with all the kleenex dust on top of the tank lid.
Besides MAP ratings, translated to concerns about flush capacity without clogging, you need to consider bowl wash. There's a very narrow but deep water spot on dual flush toilets and not all wash the sides equally effective. You can search this site or Youtube for a disturbing video of a Caroma dual flush that sprays stuff all over. Be prepared for keeping a toilet brush handy for the occassional need to cleanup after flushing... especially after having too many burittos.