In either case, I'd need to pull up the existing flooring which is NOT happening, hence why I'm going with the hydronic system. Obviously the less material between the heating source and the room the better, but everything I've read indicates that hydronic systems work very well even when installed under the subfloor. Everything I've read about electric heating mats indicates that they need to go OVER the subfloor, so they're not an option at this point.
So, this is where you're not seeing the forest for the trees. There are several basic house efficiency things that are counter intuitive here. Radiant floor heating might still be a decent option, but you are have far from approached the low hanging fruit options.
1) Heat rises
only in an unsealed system. Heat does not actually "rise". If you had a perfectly sealed box and put hot air in it, it would dissipate and fill the box evenly. If you have a leaky house, it allows cool dense outside air to push it's way in and that effectively pushes the hot air out the top of your house. Leaky houses have all kinds of comfort issues- cold floors, cold spots near walls, hot second floor and the like. So by heating your floor you are indeed creating the heat at the bottom of the living area, where it will rise up and leak right out through your roof. You've just let expensive conditioned air escape.
The fix for this is to do an energy audit. See how leaky your house is and where those leaks are coming from. Generally you can DIY the big fixes for this or farm it out for relatively cheap. What's more efficient than switching to solar panels? Needing less solar panels in the first place. This is similar to the MMM lifestyle; the less you need to live, the more freedom you have.
There are modern house designs that can heat a 1500sq foot space with a 1800W heater. That's basically heating a whole house with a hair dryer. You can't get to this level with a retrofit, but you can work your way there. Fix the bones of your house first, then add the cool gadgets.
2) As others mentioned, electrically heated water
under your floorboards is not ideal. It works, but you lose a lot of that energy to the space below it (even if insulated, there is still convection losses). No mater how you use electricity- solar panels, grid, etc, there will be conversion losses. That is why heat pumps are becoming so popular. It is more efficient to use electricity to run a compressor to pipe refrigerant around than it is to directly heat with electricity.
3) Despite that, electric radiant floors are still a good option because of the comfort factor associated with touching warm surfaces. You may find that the ambient temperature can be lower if you have warm feet (same reason electric blankets and the like work so well)
The gist of what I'm saying is that don't throw time and money at this until you get an energy audit. I personally would spend all of that money on electric floors on an even more thorough insulation job and air sealing the whole house.