For last week's Tiny Desk Kitchen, Allison Aubrey conducted a blind taste test of grass-fed and corn-fed beef. The participants, Ari Shapiro and Susan Stamberg, actually thought the grain-fed beef was tastier and tenderer, but as Shapiro explained at the end, "On the basis of taste alone, I prefer [corn-fed beef]. I would choose to order something that I knew was more sustainable even if I found it to be a little less tender or a little less beefy tasting."
Aubrey says the reason that grain-fed beef is a bit more palatable to most people is because it's fattier. "It's the difference, for humans, between eating bags of spinach all day vs. dense, calorie-rich oatmeal." I think that analogy is too soft, because cows can't properly digest corn. Michael Pollan says it's actually more like humans eating a diet of only Snickers bars. "It wreaks havoc on a digestive system that has evolved to do something quite miraculous, which is digest grass."
According to the NPR piece, grass-fed beef is generally leaner, and therefore healthier, than corn-fed beef, because grass-fed cows spend energy roaming around the pasture. Grass-fed beef is also said to have twice as much omega-3 fatty acids than corn-fed beef.