Big bullets are less dependent on velocity to work their mojo, which is why I didn't go with 5.45. With commie bullets, and a short barrel, you will get crazy muzzle flash, though. So much so, that in my opinion, it makes the rifle impractical for anything other than amusement. Blast is concussive, as well.
Just go with whichever caliber you like better. I can't speak to what platform is best to start from today, I got my stuff a long time back.