.223 is somewhat better for target shooting (less recoil, flatter trajectory) and HD (better selection of defensive loads available, less wall penetration).
7.62x39mm is more versatile (you can shoot targets with it, but you can also hunt with it). Match grade ammo and good defensive ammo is hard to find though (it's out there, but you have to look for it).
Price these days is comparable.
In the Ruger Mini platform, both will be reliable. The only downside is, you'll have to do a bit of searching to find reliable magazines holding more than 5 rounds (they're out there, though).
Now, if you start looking into guns other than the Ruger Mini-xx, then this will turn into an AR vs. AK debate, and that is a whole 'nother thread.