I'd get the 243. The 7.62 will kill deer okay within 100 yards but it was concieved as a light assault weapon round. Something that could be fired on full auto without much muzzle climb. In other words it's not that powerful. The 243 despite being very very low on recoil has about 500 more foot pounds of energy at the muzzle. The 100gr 243 bullet has a better B.C than the 125gr 7.62. Despite being a larger diameter the much slower speed of the 7.62 is going to negate any potential advantage in killing power over the 243. Also as your son gets older and able to take longer shots the energy of the 7.62 drops pretty sharply past 100 yards. The 243 on the other hand still has more than 1000 fps of energy at 300 yards. The 7.62 drops under 1000 fps of energy well before youget to 200 yards. Also while there is definately a lot of cheap ammo out there for the 7.62 almost all of it is low grade military surplus or Russian made stuff intended for plinking with AK-47 and SKS type rifles. The selection of well made 7.62 hunting bullets will be miniscule compared to whats out there for the 243.