What kind of fish will you be keeping in the tank? If you have fish that like to dig, like some cichlids, the gravel won't hold the pumice in place for long.
Undergravel heating uses "substrate heating cables" rather than a regular aquarium heater, which are rather hard to find here, and you'll most likely have to build your own DIY style (and everyone knows water and electricity doesn't mix well). Also they pretty much need to be on constantly for their advantage over regular heating to even work, and that advantage is promoting a very slight water current in the substrate or warming the roots of plants or both.
So unless you have a real need for the best possible environment for growing plants and a lot of cash you don't mind parting with, my advice is stick to conventional heating methods.