I think caliber for defense depends on your area. For urban/suburban the .223 is probably ideal except for the mags. In which case if defense is a real concern for this rifle I would consider an AR15 pattern. It has great ergonomics and optics platform, and if you stock up on mags the cost savings offset much of the cost of the rifle. Although a few ten round rotary mags in your Ruger should be plenty for any defense short of total societal breakdown. The main thing is to get some quality defensive ammo which is also very good for larger pests such as coyotes if you live in a rural area (ranch/farm lands). Winchester 64 gr SP, or my favorite Federal Fusion 62 gr PSP. The 75-77 gr HP/tipped match loads are also rated good for non-barrier defense. That is easily good defense out to 250 yds, further for a good rifleman. Hard to imagine a self defense situation where you need to shoot much beyond 50 yds., but doesn't hurt to have the option.
The .308 is a bit much for self-defense unless you are in the boonies or have lots of black bears intruding (rare), or expect to defend form vehicle mounted attack. That can be effective to at least 500 yds, so short of war or hunting large game I think it has much less use for defense than a .223 which is lighter, easier to handle quickly, holds more rounds, and has less over-penetration. If I lived in the southwest with wide open desert and possibility of drug criminals then I would probably want a .308 for stand off distance (assuming I encounter them on open range or my own land). Not a stand you ground defense but scoot and only shoot to keep scootin'.
It depends on your environment and expected threats (likelihood of occurring and degree of risk).
