I also have a .243 and have killed several deer with that cartridge over the years. In no way has it proven more effective for me than the .30-30.
Your milage may vary but a 243 with a 100 grain bullet has significantly more foot pounds of energy at any distance than a 30-30 with a 150 grain bullet does and has more foot pounds of energy at 400 yards than the 30-30 does at 150 yards. I know that energy levels aren't the only factor but the level of impact energy a bullet strikes with does directly affect things like bullet expansion and tissue trauma. More internal damage to lungs, heart and major blood vessels tend to cause more rapid death. Not to mention the 243 having a way flatter trajectory.
