I have been asking me this every year, and thus let me help you a bit.
1) Do they care about free tickets from miles? If yes then AA is the way to go. AA are throwing away miles at you and their upgrade policy is so lax and easy to use. Also less miles to earn reward CX F and J flights. HOWEVER, please do note, as by others, that only Y, B, H classes are AA eligible.
2) Do they care about service? Then CX is the way to go. Of course there are people here that insists US-based airlines are better at it, so you can experiment if you want.
3) This is, personally, a big one for me. It is clear CX really takes care of their DM and GO members. I have many small incidents that were ignored with just ok service in the past, but now as GO I can really sense every request I made that CX staff will do all they can to make it happen. I really appreciate that.
There you have it. Which is more important: miles to spent, good service on flights, or really good personal service as CX's DM and GO members?