I don't have either. I've shot a 6XC for a while(another improved 243).
Just looking over the 243AI -vs- 6SLR
The 243AI has a 40deg shoulder and lower body taper than the 30deg 6SLR. It also holds a little more capacity. This would provide for better brass life, but at the further expense of barrel life(243 is already a barrel burner). 6SLR improves barrel life and brass life(a little)over a 243.
There may have been exceptional loads found with the 6SLR & this might tip the scale.
Another similar is the 6CM, which has made a lot of competitors happy.
6CM testing Results