Thank you all very much for this help. I'll definitely look in to the sign-up bonus for the airline credit cards, but they seem to be cracking down a little and demanding you spend on them -- which I'm not willing to do.

From the guidance you folks have given me, I think I now know what my own personal rules of thumb are.

I'm not particularly fussed about upgrades in-flight. I generally just hunker down, bring a sandwich, and read a book or work (or watch a movie on the laptop.) That means the perks of that sort of thing generally don't mean much, so it's not worth me making a special effort to concentrate on one airline to build up segments.

Meanwhile, until you hit some pretty high levels of status, it's tough to get much of a discount on first-class lounge entry. Generally, you are required to get a membership -- which means being locked in to a particular airline.

What that means in general is that I'll be using miles just to gain additional travel tickets. If I can save even $50 a flight by splitting between United and American (generally not hard), it makes sense for me even if I lose on gaining status. I can just use the $50 to buy a one-day pass to the lounge. A perk shortcut, really, that's hard to get the "earned" way.

So in the end, my personal rule of thumb is "if there's less than $50 difference, pick the preferred carrier" -- (mine is American) -- "otherwise, take the cheapest of United and American and squirrel away the miles for later years."

Thanks again folks. I don't know what people did before the internet.

PS: Per suggestion, I looked into that special pass that gets you in to a bunch of lounges. It might actually be the best deal for me, but on the other hand it doesn't work for all lounges and I noticed that both United and American aren't served (at least in my current home base) making things less convenient. I think what I'll do is watch my usage this year, keep an eye out for other lounges, and assess at the end if a pass it worth it.

I'd be interested to hear if anybody has bought the "Priority Pass" and found it better than going with the day pass!