I am Plat on CO and lifetime Gold on AA, and Gold on SQ. In making your decision, AA will actually let you use your miles, and it is almost impossible on CO--they prefer you use them on some other carrier. I personally think CO has the best service, but the worst leg room. The other matter to consider is that you have some control on upgrades on AA, whereas CO's liberal policy of upgrades will go to Y class and Plat's first, so your silver and gold days will be in coach.

PFG