Your're kinda looking at two different applications...You mention competition and target practice...Forget the .270 for that as most .243s and .223s when tuned will out shoot a .270...
As a general rule the .223 will outshoot a .243 in addition it would be more pleasant to shoot if you are talking about shooting a hundred rounds a day...Add in that it's cheaper to reload...
Since you have a .308 I'd go with the .223 as it's actually a few steps up from a .22 mag...