If you can avoid United, do. Despite all the claims above about "IRROPS" being better on United, keep in mind:

1) they're more likely to occur on United
2) you're more likely to encounter nasty, surly, and utterly unhelpful agents and employees on United when you try to fix it.
3) United is more likely to claim that a mechanical or other problem that was their fault, was actually due to circumstances beyond their control. So you the passenger are screwed.

Take it from a United GS member (just about the highest status you can have on United), I've often gotten lame treatment on United in "IRROPS," e.g. agents who didn't want to rebook on the best alternative because a cheaper fare class wasn't available.