AA and UA at the top tier (100K) are very different. At AA, you get 8 systemwides good on any fare, subject to inventory. At UA, with 6 systemwides, you'd have to buy up to a "W" fare or higher on international and this could be hundreds of dollars, depending where you're going. If you don't clear the upgrade at UA, you don't get any of that additional fare back.

My sense the last few months is that UA flyers are not doing as well as AA flyers on domestic upgrades, and that may be due to the very visible UGS tier that is above 1K at UA (a high-revenue tier) that would clear upgrades first. I've seen some UA flyers at SFO reporting 20-30% domestic upgrade rates while I sit at 91% at AA.

Also, does UA have much of an operation at LGA? Are you prepared to get over to Newark which offers a lot more options, or would you prefer AA at JFK for more flight options?

Another consideration is that partner awards on UA, particularly in business and first class on international routes, cost more than flights on UA metal. At AA partners have no special pricing so you could fly AA or Cathay to Hong Kong in first class for the same amount of miles.

At UA you will earn 11 miles for each dollar spent. Plot out your planned 100,000 miles and see what you earn there, and then plot it out at AA where you will earn 100% flight miles plus 100% bonus miles. A trip from SFO to Europe typically brings in 24-25,000 miles depending on routing. For an $800 ticket on UA, after you take the taxes away, you'll bring in about 7,700 miles, so there is a big difference in miles earned. You'd need to buy the $2,200 ticket at UA to bring in the same miles AA would give you at the 100K level.

Change that to a $300 transcon and AA brings in 10,000 miles for you, while, at UA, after taxes are removed, you would bring in around 3,000. You'd need to spend around $900 before taxes to bring in 10,000 miles at UA.

Not sure what you mean about getting status with a credit card. Do you mean some credit towards the 100,000 miles?