I think both UA and AA have the same "rule buster" type awards (usually for 2x the regular award level). I know I've booked them on flights that were nearly sold out. I believe that's the intent: if there's any seat available, you get it by paying double for it.
I fly both UA and AA. I think both are quite fair with award availability and the rules on awards. In my anecdotal experience, I have found UA easier to book to Europe and AA easier to book to Hawaii, Central/South America, and Canada. I find UA easier to book on regular domestics (e.g., MCI-DEN-West Coast), but AA easier if I need 4 award seats on the same flight.
AA has the a bit of an advantage in that one elite can book four travelers together on one itinerary and get assigned seats for all with no problems. As a UA 1P, I had to use my 3P assignment for my wife, and now when four of us travel we have to book it under two itins. Not a huge deal, but people who don't know this and goof it up by booking 4 together get
no sympathy from UA whatsoever. It isn't well-explained...it's just something you learn by reading these boards and learning from others.
It's nice to have healthy balances in both. Both have their strengths/weaknesses.
I always had great experiences with US back in the '90's when I was a GP, but now I have about 60k left on them and haven't found
any availability to use the miles when I've wanted to in recent years.
DL seems to be an odd bird in that I can always get international availability on them (including, last year, three 35k seats to Belize in max-peak season), but I have the most difficult time getting the simple MCI-ATL segments to clear, even on wide-open flights. Waitlist, waitlist, waitlist....hope they cleaer, usually they eventually do...but there's always this lingering fear that I'm going to be buying a last-minute AirTran ticket home.
CO is a wreck. I guess it's a good program if you want free train rides on Amtrak.
I use NW for one reason: MCI-DTW runs (we have family there). Fares on this route are absurd, but 25k awards are usually not too hard to find. One of those rare domestic routes where all flights can be $450 R/T & up, but all of the cheap fare buckets (and awards) are wide open. I know little about the rest of the NW program.