If you don't have any driving purpose then get the pistol caliber carbine, PCC (assuming that is what you meant). Especially if you already own a Glock or Ruger 9mm pistol. It will use your current pistol ammo which is cheaper and not expanding the types of ammo to stock up on. It will use common pistol mags so again no need to stock yet another type of mag. It will be fun, relatively cheap to shoot, can shoot at indoor and outdoor ranges, and can be used for self defense. May or may not be useful for certain types of varmint control.
What might drive one to consider the .223 carbine/rifle? Longer range for sure, either targets or varmints. And more accurate. More powerful than a handgun caliber so more effective in self defense and still suitable for defense in the home as much as the PCC is. If you served in the military it will be more familiar manual of arms, field stripping. Also, the M4 carbine has tons of aftermarket parts and upgrades so if you work your rifle hard then it might be easier to maintain. Plus, AR mags are even cheaper than Glock mags!
The PCC is mostly a near-to-100 yd firearm, whereas the Ar15/M4 is a near-to-600yd or further firearm. Just for fun? The PCC is the less expensive to feed (a few mags and lots of ammo). Putting it to hard work on range, varmints, or defense/training? The AR15 is well proven in that role and has many more options for configuration from lots of quality sources, and cheaper to buy lots of mags.