I chose the 6mmAI and went with the Norma 7x57 brass. My Hart barrel had a 12" twist and shot very well with 55gr-80gr bullets. The 55gr B-Tips can be (safely) shot so fast that they are flatter, and resist wind better, than the heavier bullets all the way out to about 750yds. After that the heavies take over.
Fireforming can be a pain in the neck. But if you're comparing the 243AI to the 6mmAI, then you would have to fireform either way.
The 6mmAI does have a moderate velocity advantage over the 243AI, if that's important. In a custom rifle
either cartridge is capable of superb accuracy.
I hope I haven't just made your decision a little harder. [img]images/icons/rolleyes.gif[/img]