Yeah, it is, but the x39 IMHO just doesn't have enough more on it, to make a real difference in the end for most applications.
I shy away due to the magazine fact, though my nephew has a few that work flawlessly.
I'd rather take it based off the 223 since I have probably close to 100K cases that I could play with also.
I also consider my 6 woa to be a 243 light so to speak, I've done everything with it that I've ever done with a regular 243 and they are more apart than the x39 vs the AAC. X39 with a 308 bore really doesnt' have that many flaws.