The "hydronic" one:
is filled with silicon oil. I'm not sure who the marketing guru was who thought silicon=hydronic but whatever.
The other is likely a plain, nichrome resistive element type (like a toaster) with no silicon oil/"hydronics".
Thermodynamics being what they are, you can't get more energy out than is put into a system. So, for every watt of energy spent heating up the oil, those can't be used for heating the air in the room. Intuitively, it seems to make sense that the oil would "retain" the heat, but, again, what is put in, is taken out. Otherwise, this would be akin to a perpetual motion machine; just heat it a bit, and it continues to give heat longer than it should. People on Amazon who have them and reviewed the silicon one seem to like it. How big is this room you are trying to heat? Where do you live? How hot do you want it to get? etc.....Children involved?