IME you can check your baggage limit for the international segment, so go for the 3 at 23k for J an LAN. I frequently do LAN from SCL-LAX-DEN, and there's no F on the AA flights from LAX-DEN (it's a CRJ, for crying out loud...), and never had a problem with the bags.
You'll enjoy LAN's J much more than AA's. There's a lot of comparisons out there already under the "American Airlines" and the "Other North and South American Program" discussions. 2A is great - just remember that it's tough to get to the aisle when 2B is fully reclined; I always go for 2D or 2H.
For Robt760: No special service considerations like FEBO on AA...they start at the front.
Currently LAN arrives at Terminal J in MIA, and AA operates out of D and E. You'll have to collect your bags, clear customs and re-check them before leaving security and heading towards E and D. It's a bit of a walk - allow 15-20 minutes ish.
Take the time to look for Premium Passenger access. I don't know if it exists at E and D, but it will be worth it if it does (the AA website neither confirms nor denies the existence of such). I missed it when I went to my connection to LAN at J and discovered that the other pax breezed through security while I spent 45 minutes+ in line.
Once through security, you'll have access to the AC (based on the QF status in your profile). I just wrote up my impressions of the two ACs in D; I have never been in E to comment. Go to D30 over D15 if you have a choice. My impressions are here:
http://www.flyertalk.com/forum/ameri...l#post15400057
AA economy isn't bad, but remember that if you want to purchase a drink or food you need a credit card - no cash (unless your flight is American Eagle). It's not that far to DCA, anyway...
Happy travels!