If the goal is fun, then get a .223/5.56 as you will be able to shoot more from a cost perspective and an endurance perspective.
If your goal is Heavy Metal type division, then go with the .308.
If your goal is to kill larger animals, or medium animals at long distance go with the .308.
If you want a .308 just because you think it's cool, don't ask anyone, just buy it.
However, if you want a .308 that has the option of becoming a .223, you might look at a Colt 901. But you are going to pay more for the Colt. Another brand of .308 you should look at would be Noveske.