Carbon fiber is really nasty if you try to sand or machine it. It will tear up your lungs and if any pieces get under your skin you’ll have a very tough time getting them out.
I have a slightly different take on this.
Just casual observation, but Carbon Fiber AR15s seem to be slightly easier to find these days. Personally, I would rather have a Carbon Fiber AR15 than no AR15.
If you can find one, go for it.
A close second is a Mini-14, which will fire either .223 or 5.56.