I love your idea of radiant, hate the NG though. But I guess it would come down to install pricing, seeing as in Mass you can't legally instal either system, nor buy the pieces. I honestly believe oil would be the best bet. Not to mention people usually grossly oversize their tank(my dad) and run WAYYY more than they have to. You're supposed to have a tank that will hold 1/3 of your yearly consumption. My guess it you'd need one 250gal MAYBE a 275 if you liked it really warm. But check out the lower volume wall mount tankless water heaters. Even I'll say those things kick dick with gas Aslong as you're not trying to pull more GPH through it than it can push, you'll never run out of hot water. Even if you do pull more water, it will only be gradually cooler.
As you can tell, I'm what my refrigeration teacher calls "A XXXXing heating guy"