x39 traditionally has feeding problems out of AR-pattern rifles. This is due to the taper of the case, and the (straight) shape of the AR mag well.
In regard to the price of ammo, cheap ammo will tend to be inconsistent and therefore inaccurate... this is the hallmark of the x39 round. Quality x39 ammo isn't any less expensive than quality 300BLK. If you're going to stick with the AR platform, I'd go with the 300BLK for the ease of feeding/functioning alone.