Go with the 223. Ammo cost next to nothing and you can shoot it all day without getting a sore shoulder. Fire 100 out of the 223 an you won't notice it at all. Fire 100 out of the 308 and you will feel it the next morning I promise you. Firing the 223 will make you a better shot with your larger caliber rifles.

When I was starting out I used a 222 until I was proficient. Then I graduated to a 270 with no problems. If this is any indication I'm usually the one my buddies bring their magnums to for sighting in. Luck me.