I switched the bulk of my flying to AA three years ago, but still fly CO enough to make plat. IME, if you are not buying full Y fares, you will tend to be upgraded more on AA if you are an EXP than you will on CO as a Plat. There are several reasons.

CO requires only 75,000 miles or 90 segments to make Plat while AA requires 100 segments and 100,000 miles or points to make EXP. CO grants double segment credit for its highest coach fare levels, including H and K fares, while AA does not grant any double segment credits, even for seats in full fare first class (They do grant 1.5 points for fares from Y and B on up through all business and first fares. CO recognizes higher fares by granting 1.5 qualifying miles per each mile for higher fares.) With double segment credits, a flyer can fly relatively cheap fares on CO, especially on routes on which WN competes, and make Plat on CO with flying that would leave the flyer a mere Gold on AA. Twelve round trips from AUS or SAT to MSY through IAH will create a CO Plat, the highest level, by flying some coach fares below Y, which such flying would leave an AA traveler flying such fares as a Gold, the lowest level

Second AA has more overall inventory in First, IME. The old MD-80 have more seats that co's workhorse 735;'s and 733's.

Third, Co makes all Y fares Y-up fares, any Y fare will get you into first domestic or in North and Central America and even on a few routes out of IAH to northern South America as long as there are seats in the F cabin. Not all of AA's Y fares are Y-ups, some will leave you in the upgrade queue.

OTOH, if your employer will pick up the cost of any "coach" fare, CO is a nearly continuous stream of upgrades. I know of several flyers who "comply" with corporate policy, show a receipt for a coach fare and use Y-ups on CO to stay out of coach.

Coach catering is not at all exciting on CO and is mostly buy on board on AA.

While AA's fleet is older, I don't notice a substantial difference between the two overall. Planes are dirty, period. Don't fly if you expect hospital, or even restaurant cleanliness levels.

If you can get any coach fare paid for you and upgrades are important, go with CO. If as I do, you have to pay your own way most of the time and upgrades are important go AA. I can only compare EXP on AA with Plat on CO, so I cannot give you a truly informed comparison at other elite levels