Yeah, don't be afraid to fly United because of upgrade potential. The way UA and CO work is they decide at any given time how many seats they are going to allocate for upgrade (ie. that they'll confirm with a Regional, Systemwide, or Complimentary Upgrade within the window). If you want to see this, when you're searching for flights on CO.com, click fare class and look at the number next to "R" for the number available. They only put seats in the "R" category (or NF/NC in UA's case) if they think they won't sell them- why give it away for free if they can sell it? So, your varying experiences reflects CO's varying predictions of whether they think they can sell seats or should just give them away.