GoCanes, As Rssrsvp said it does depend a lot on travel patterns and what you expect from elite programs. Is it upgrades, miles, lounge benefits, a mix of all, etc.

There are too many threads comparing various elite programs. Best to look at summary charts (use www.fewmiles.net for helpful links).

From international lounges OW and Star networks are the best and therefore you get better global lounge access. Note that for most of the OW/Star partners you need to fly on a OW/Star carrier on an international segment to be able to use the lounge (some airlines give you access to their lounge no matter which carrier you fly if you are their super elite).

As for miles - AA PLAT get 200% bonus effectively (100% on most OneWorld flts and 4 500-mile electronic upgrades for every 10,000 miles flown which can again be converted to 10,000 miles if your travel is mostly international).

Coming to upgrades - AA and UA upgrades are limited unlike CO/NW. AA and UA allow upgrades to lot more fares than CO/NW.

Comparing just AA and UA - I have heard from colleagues that mid-tier on AA is better than mid-tier on UA for west coast travel. I am EXP on AA and have been 1K or PremExec on UA. I found that UA upgrades were mostly gone well before the last minute while AA blocked some upgrades until the last minute. Even on paid F and J I had to downgrade to coach on UA (as F seats were not preserved for last minute travellers) while on AA I have never had that problem.

A friend who flies NW a lot (based out of DTW) finds NW/CO elite very rewarding but I find that outside of DTW it is not that attractive.