Out of those choices I'd go with the 6.8spc since I have a bunch of 223 and 556 uppers already. Since you're really only talking about the upper with those, why not build one carbine stocked and one A2 stocked rifle lower. To swap out the uppers just takes a few seconds.
I've been toying with this same question for a couple of months now.
I have a 16" CAR and a 24" varminter in 5.56 already. I also have a 3rd complete lower just waiting for me to decide what upper to hang on it.
Have any of you considered this? It's really piqued my interest.
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