"as a low level elite, i doubt you'll be getting many "bump-up" upgrades. when the economy cabin is oversold, the airlines normally start at the top of their ff status list and work their way down so the few upgrades are normally snatched by uber-elites."
Not hard to tell who he flies on. Some companies upgrade people comestically without being oversold.
Unless you fly 100K+ and really care about the ability to be upgraded internationally, NW is really your best choice with CO being second. Flying on NW regardless of fare you'll get 100% EQM regardless of where you book it. You'll get upgrades on NW where available and also upgrades on CO where available. You can fly NW, CO, DL or AS and get points on all of them. The number of EQM will vary. You will get the upgrades on NW/CO once you get to any elite level. Basically, when they have the space, they start upgrading by status/fare. Also, getting a lounge pass for World Club will let you in the lounges of each of those airlines.
In terms of NW vs CO, the former gives you a few benefits. NW gives a better upgrade percentage, at platinum they give miles if you don't get upgraded, don't require booking at NWA.com to get 100% EQM and have better offers than CO. You also get to upgrade on AS with NW.
I would not go DL. You won't get upgrades on CO/NW as an elite. Only on DL.
Flying internationally if you fly over 100K EQM per year then UA or AA are better choices. AA will automatically upgrade you domestically if you get top status, but that takes 100K and you still pay for your own drinks in their lounges. With over 100K EQM per year, they will both give you system wide upgrades that you can use internationally.
"I love United's system but man they're FF program looks ugly compared to the rest unless you fly full fare all the time....no comp upgrades even for 1k on cheapy fares, are you kidding me!!"
They give you those upgrade coupons. Mine used to expire before I got to use them. If you don't have enough, you have to buy more. I've flow on UA plenty of times as a gold elite and had many seats open up front while I sat in back tired, cramped and pissed off.