You already know Hank Hill's answer: gas. That's also what Wired editor Mark McClusky thinks. He argues that "grilling over gas is scientifically, objectively better than grilling over charcoal." McClusky explains:
And radiant heat is what’s really cooking your food on a grill. That’s why gas grills use some sort of surface to create radiation, whether it’s lava rocks or ceramic plates or the “Flavorizer Bars” on my Weber. These surfaces are heated by the gas flame, creating the radiant heat generated naturally by charcoal.
Charcoal purists will try and tell you that their preferred fuel leads to better flavor. This is, well, nonsense.
Your food doesn’t know what’s creating the heat below it, and once charcoal is hot, there aren’t any aromatic compounds left in the coals. According to the food science bible Modernist Cuisine, “Carbon is carbon; as it burns, it imparts no flavor of its own to the food being grilled.”
The characteristic flavor of grilled food comes from the drippings, not the fuel. When those drippings hit the heat source below, the oils, sugars, and proteins burst into smoke and flame. That heat creates new complex molecules that rise in the smoke and warm air to coat the food you’re grilling.
Nothing in that process relies on charcoal.
But Joe Brown, the New York editor of Wired, takes the opposite view:
What charcoal brings to the party is a healthy heaping of aroma compounds, the other half of the power couple that is flavor. In fact, aroma might be the super starlet in that relationship, because our tongues are actually pretty limited. “There are only five taste receptors that are well-agreed-upon to exist within your taste buds,” says Sacks. He’s referring to sweet, salty, sour, bitter, and the new kid: umami.
Anything else you perceive while eating — that smoky deliciousness, for example — is courtesy of aroma.
Aromas are released when you bite into your food. They travel up your retronasal cavity, and light up your olfactory receptors. That neurological signal mixes with whatever your taste buds are saying and tells your brain what’s going on in your mouth.
Of course, even food cooked on a gas grill gives off aromas — all food does. But food grilled over a charcoal flame has a special one: guaiacol.
Guaiacol is an aroma compound produced when you use heat to break down lignin, the resin responsible for holding strands of cellulose together to form wood. “It has a smoky, spicy, bacony aroma,” says Sacks. “In fact, the flavor that most people associate with bacon is largely degraded lignin.”
Translation: Cooking over charcoal makes your food taste like bacon. Let me repeat that: blah blah charcoal blah blah BACON.
-via Glenn Reynolds
(Image: Grill Sergeant Apron, now on sale at the NeatoShop!)
Arguing the wood compounds in charcoal add a lot to the flavor seems to me like someone arguing grain alcohol is better than water because it has some traces of the grain left, but ignoring whiskey.
Charcoal is a must, however, when smoking meat. I'm a snob and only use natural charcoal and a chimney starter.