don't they make a mini-14 which is for 7.62X30?
which caliber is better?
You probably mean 7.62X39. It is probably better for deer hunting or if you have access to a lot of that ammo cheap.
For self defense I would probably rate them about equal.
It is called the mini-30.
IIRC The Ruger Mini-30 is chambered for the 7.62x39.
which caliber is better?
As to which caliber is "better"...
First, you should know that I'm a bit biased. I own four rifles chambered for the 7.62x39 cartridge - 2 SKS rifles, a WASR-10, and an Interarms Mark X (a bolt-action "mini" mauser - my deer rifle).
I own zero rifles chambered for the .223 Remington/5.56 Nato.
Which is "better" is also relative, and depends on what the rifle will be used for.
The .223 Remington has a higher muzzle velocity (and less bullet weight) than the 7.62x39, so it will have a flatter trajectory.
However, the 7.62x39 will deliver more foot-pounds of energy to the target.
If you want to compare the ballistics of the 7.62x39 & .223 cartridges, there are two ballistic programs I recommend (assuming you aren't already familiar with these):
The first is free: Remington Shoot! (Limited to Remington ammunition.)
The second is shareware (30 day evaluation): Shoot! v3.0
Shoot! v3 includes data from most (if not all) ammunition manufacturers. You can compare any three cartridges (any caliber, bullet weight, etc.)
You may also enter your own load data, if you wish.