Don't overcook the porterhouse for best results imo.
The porterhouse is from the Tbone, it's the bigger piece of meat on the Tbone, the smaller piece is Eye fillet.
Grass/Grain much the same imo, grain might be more tender but isn't always, some say different flavours between Gras and grain fed but i think it's hard to guage, they stick grass eating beef on grain for the last 90 days or so to fatten them up.