As a member of both programs, I think the answer depends on how much you're going to fly in a year. It is difficult to get to DM on CX starting from scratch, where as it is SIGNIFICANTLY easier to get ExecPlat or OW Emerald on AA.
For example (just using BIS miles for simplification), assuming you just renewed CX/SL, it will take you 60K to get to GO and then another 120K to get to DM. So 180K in all.
On AA, you need 100K miles and you're ExecPlat.
Earn and Burn is also much more favorable on AA.
My strategy would be:
-If you think you can make CX Gold, go for CX if you care about op-ups; go for AA if you care about earning and redeeming miles.
-If you think you can make CX Diamond, go for CX if you care about op-ups, but think hard about splitting miles to get AA Execplat and CX Gold; otherwise, if it's earning and redeeming miles, skip CX and go for AA.
-If you can only pull SL, then put everything onto AA if you can make AA Platinum, otherwise keep in CX.
All this assumes you're flying the right fare classes to earn AA miles.