The Carbon 15, if it is one of the new Bushy's, uses standard AR 15 bolt and bolt carrier, is lightweight and a nice rifle.
The AR-180B has a better gas system, uses non standard AR parts, and is lightweight also.
I think it would be easier to scope a Carbon 15 than it would the AR180B.
I own a Carbon 15 pistol and it is a lot of fun to shoot, maybe my favorite pistol of all. It shoots a huge fireball and is very loud. But I think the .223 out of such a short barrel is ballistically worthless, akin to a 22lr in terms of stopping power. I have had no problems at all with my Carbon 15 pistol, when I found which mags it likes it works perfectly.
My cousin has the Carbon 15 rifle and his works OK. He sent it back to Professional Ordance because it kept jamming. Now it runs good, but the trigger doubles. But when they work good, a Carbon 15 is hard to beat for a nice lightweight rifle.
So in the end I would probably get a Bushmaster Carbon 15 just because it uses standard AR 15 parts, and has a better method for mounting optics.