Well, if it is only domestic travel, I would say to avoid UA (and I'm a proud UA'er!). If there is international travel, then UA is much more attractive as *A has the most depth of international partners, with AA second, and SkyMiles way below that (IMO of course). I

If you're going to fly the airline (and not their alliance partners) a lot, I really like AA, then DL. If you're going to fly the partners a lot (for intl travel), then UA is the best IMO. My reasons are in the last paragraph below which you can read if you'd like.

So, go compare AA and DL. One thing great about DL is you only need 75K EQM for the top level, where as AA requires 100K for EXP. SkyMiles looks to be expanding as well.

However, redeeming on DL appears to be a RIP OFF!! Their chart shows one-way, not return fares. The numbers are insane compared to UA and AA!! If using miles for international travel is your goal, AVOID DL!! 100K for US-AUS roundtrip in the "low" end. 60K roundtrip US-Europe in the "low" end, going to 90K at medium. Maybe award availability is better with DL, but the mileage prices are much higher! Your value per earned mile is not great on DL. This is my perception, but perhaps others can change my opinion.

I'd go AA. If she lives in NJ, then CO is great out of newark.

My own reasoning on UA being great for intl travel w/ partner airlines:

I've been Platinum on AA for a few years and am 1P on UA for the last 4 years. The ONLY reason I switched was because AA completely DEVALUED the miles earnings (both EQM and RDM) of partner airlines. So if I flew Qantas or BA or Cathay (all OneWorld partners), I'd get CRAP in return unless I was in an expensive fare class in E (25% EQM for BA, 50% for QF, and one leg from Hong Kong to Aus I got ZERO EQM with Cathay). I tended to fly BA and Qantas a lot since I'm in Australia and flew to both Europe and the US. With the BIS miles it would take to get Platinum (50K EQM) on AA, I could get 1K on United. It was a complete no-brainer for me.

If AA changed its partner earning for discount economy tickets, I'd likely go back. United gives 100% EQM for discount economy tickets on SingAir and Lufthana, and I put a good # of miles on both each year, but majority of my BIS miles is on United.