I did a little grid on another thread that shows "L" class benefits on various carriers sorted by FFP memberhsip. Here it is:
Booking Class L - Redeemable miles/km per mile/km flown
Code:
FFP> AA* AY BA CX EI IB LA QF
*Pts *Pts
FLY
V
AA* 1 1 0.25 1 Y Y 1 0.5
AY** 1 1 0.25 1 Y Y 1 1
BA* 0.25 0.5 0.25 0.5 Y Y 0.25 0.25
CX 0 1 0.25 1 Y N 1 0.5
EI 0 1 0.25 1 Y Y 1 1
IB 0.3 1 0.25 0 Y Y 0.3 0
LA 1 1 0.25 1 Y Y 1 1
QF 0.5 1 0.25 0.5 Y Y 0.7 1
AY Dom 1 0.5 0.25 1 Y Y 1 1
LP I/N 1 1 0.25 0 ? ? 1 1
LP Dom 0 0 0.25 0 ? ? 1 0
* No AA credit on BA US-LON, no BA credit on AA US-LON and v.v.
** Most plans exclude AY "Excursion" (Y-only) flt nos.
*Pts - FFP uses distance-based points; No./pct varies by class/distance
At sub-elite level it looks like AY's might be more generous that AA's; however redemption requirements are extremely important, as is specific itinerary. It does look, though, like the OP could hit Platinum somewhere around Cozumel using the challenge, in which case AA would probably be the preferable plan.