What's the single most important priority Congress needs to address vis a vis healthcare reform? The two obvious answers are controlling costs or covering the uninsured, but how can you decide between the two? Covering everybody without controlling spending will lead to rising costs. Controlling costs without covering the uninsured is simply unethical. Congress can — and should — draft legislation that does both at the same time.

The good news is, covering everybody is the easy part of the problem — at least in theory. All we have to do is find the money to do it. Of course, that's hard to do politically, as the heated debate over public versus private plans is demonstrating. The bad news is, controlling costs is really hard, because it requires making changes in the way that doctors and hospitals deliver care.

And getting a grip on spiraling health care costs is being made all the more contentious and confusing because many people — including pundits on op-ed pages — keep making two fundamental errors in their analysis of the problem and its solutions. First, many people assume that we spend a lot on health care because it's expensive. That might sound pretty obvious, but in this case "expensive" means "costly per unit of service." That would suggest that the way to rein in spending is by forcing hospitals, doctors, drug companies, CT-scan manufacturers and the like to accept lower prices for their goods and services.

Republicans want to let the market, and competition, do the work, which usually means making patients pay more out of pocket and shop around for lower prices. (I can see it now: very sick patient sits up in hospital bed and says, "Just how much is that MRI going to run me?") Democrats, on the other hand, generally want Medicare to do the negotiating.

Regardless of who is trying to get the better price, the patient or the payer, price isn't our biggest problem. Sure, forcing prices down would reduce spending by a bit, at least temporarily. But it wouldn't address the more important driver of our gigantic national health care bill, and that's the huge amount of unnecessary, and potentially harmful, care that patients receive, depending upon where they live.

Here's the funny thing about healthcare spending: the dramatic and relentless increases we've seen over the last three decades are due in large measure to the chaotic, inefficient and often harmful way care is delivered in a handful of places in the country. One of them is McAllen, Texas, which some readers may know about from a recent article in the New Yorker.

The Dartmouth Atlas, which many readers may have heard of, has pinpointed a dozen other parts of the country where we spend a lot and don't get particularly good care: Los Angeles, downstate New York, big chunks of Texas, Miami and the Washington metropolitan area.

If we want to put a lid on costs, we need to go after excess spending in these regions of the country. Doing so will make it that much easier to find the money to cover the uninsured. My answer to the question about priorities is cost and coverage, which are a little like soup and sandwiches and love and marriage — they have to go together.

Shannon Brownlee is a senior fellow at the New America Foundation and the author of "Overtreated: Why Too Much Medicine is Making Us Sicker and Poorer." She wrote this commentary for The Washington Post.