I'm not saying the 1:9" won't shoot the heavier bullets, more than likely you'll be able to find a load that would shoot fine, but there have been many, many members that have tried them, using conventional wisdom that the faster twist shoots the heavier bullets better, but apparently due to the very shallow rifling of the 1:9", it's not necessarily true, that's the general concensus in past discussions anyway, for instance, it may work fine with lighter charges using slower powder for a handloader, but factory ammo wouldn't work.

If you choose to get the Superlight 223 with the light weight 20" barrel, you'll be getting a 1:12" barrel until they run out of em, as of the last I checked, that's all they have ever had in it, but when they run out of em, it too will be 1:9" on the next barrel run.
Tim