243 is a fine round and would be my suggestion between the two. The heavier 6mm bullets will easily kill if the shooter is proficient and the shooter knows where to place the bullet (as most of you know, this is true for all rifle cartidges) but there will likely be way more bullet/cartridge options with a 243 at common sporting goods stores.

And I agree with GoatKiller on 30-30s. A dirty-thrity would also be a good starter rifle for a youth.