I only have experience with Delta and USAir. I tried CO just as they were merging, so I don't think that is an accurate evaluation.

US tends to be better for most flights, as they have fewer elites to compete against. That being said, their planes have fewer F class seats, so popular routes (i.e. transcons) are a little bit harder. AA has very few F class seats on their planes, but they use a coupon system. You pretty much know when you are going to get an upgrade, but it will cost you! DL tends to fly larger planes with more F class seats. This works to your advantage when you are flying unpopular routes, but because they have so many elite and are such a big player, and hub to popular destination (such as SFO or LAX or JFK) you will be pretty low on the list.

So yes, it does matter a lot on where you are flying. DL in particular is also susceptible to time - lots of contractors fly Sundays and Fridays, so those days give terrible chances at upgrades on DL. I think US tends to get fewer of those, so I notice day does not make as much a difference.