I recently discussed the hypothetical choice between more stimulus and health care, and wound up with a quite interesting comments thread. I stuck to stimulus vs. health care, because the "choice" that is being discussed today -- health care vs. climate -- seems to me mostly unrealistic. Matt Yglesias (who would have preferred climate legislation if given a choice) sums up the situation nicely: Barack Obama and the Democrats in Congress were pushed by the Democratic party coalition to put health care first, but at any rate the combination of a rejectionist strategy by Republicans and regional differences among Democrats made a climate/energy bill a longshot at best.
The one thing I'd add to what Yglesias says is just that, at least as I can see, health care really is a far more ideological issue than is climate. Fanciful rhetoric aside, the health care reform debate really was about a "government takeover" -- not of the health care industry, obviously, but of health care as a government obligation. Establishing health care, in Ted Kennedy's old phrase, as a right, and not a privilege, really is a big deal; whether or not it's an expansion of government, it's a serious and important expansion of governmental responsibility, and that surely hits along a core liberal/conservative line. In other words, while it's easy to imagine Republicans modifying existing health care programs to achieve goals that Democrats might agree with, it's impossible to imagine a Republican House, Senate, or president agreeing to anything close to universal health care. I really don't think climate works that way. It's easy for me, at least, to imagine a situation in which conservatives and liberals agree that preventing climate change is a legitimate responsibility of government, and then finding some mechanism (and set of goodies for a winning coalition of interests) to make it happen. (Yes, that does assume that ideology matters at least to some extent to at least some of the relevant actors).
So, if all that is correct, then it makes sense for Democrats to use their rare supermajority on health care, which they can get in no other way, rather than on climate, which can be achieved in different circumstances.