I have been lurking on the AA thread for a very long time. I asked for a comp to AA plat as I like their intl upgrade policy. I still like UA.
Both AA & UA are very good choices. The AA upgrade policy on intl tickets are a little better than UA. Using miles, one can upgrade the least expensive tickets whereas to upgrade on UA using miles, one has to purchase a H or above fare. On both UA & AA, one can use the Star & One world lounges on same day intl itineraries if one is mid tier or above. (UA Prem Exec & AA Platinum--50K miles/year).
Do a search on the AA & UA threads and you will find alot of info.