martyman25, welcome to the board!

As others have mentioned - do not 'expect' upgrades. Unlike AA, these are not space-available, rather only in cases where the cabin is overbooked. That said, CX does overbook a fair bit.

AA has far better earn-burn ratios and status is harder to come by on CX. You can earn AA miles by booking the same flight as AA codeshares, or in the higher fareclasses (if booking under CX flight number).
On the flip side, being a CX elite will give you accecss to AA lounges on wholly domestic-US itineraries with AA. Overall the experience in Y on CX will be better as a CX DM, rather than as an AA Exec Plat.

Also, CX's seat guarantee in coach on "V" and higher fares (@72h notice for GOs and 24h notice for DMs) will come in handy, especially during peak travel periods - might be a factor if cost is a real issue. (And the likelihood of an upgrade will be higher then as well).

If you do move over, there would be no harm in asking CX if they can give you some sort of status to start off. AA Exec Plat + travel history with CX (show that to them), should get you atleast silver (which gets you lounge access when travelling CX, and excess to priority seats), and if you're lucky maybe GO. But you will get these benefits in any case as an AA elite.