I recently went through the same decision process. It was time to replace my '99 EB 4x4. I was considering a Yukon XL and Expedition EL. The main reason I didn't go with the GM product is materials used in the cabin seem sub par to those used by Ford. Especially the leather.
My father in law bought a used loaded 2007 Yukon with low miles. The day he bought it, he brought it over. I was messing around checking it out and was surprised at how cheap the interior seemed to be. In fact, one of the plastic headrest buttons flew off when I touched it. I've ridden in it several times since and still consider the interior sub par.
When I drove the '11 Expedition EL, I was really surprised at how awesome it was on the highway. I've had it since March and I'm still excited about it. In fact, we just got back from a trip to NH, 15 hours away, and didn't feel the fatigue I've had in other vehicles. So far, the power concern I had hasn't cropped up. I didn't realize at the time that the 5.4 of '99 and the 5.4 of the newer trucks was that different.
In a nut shell, I was open to both but stuck with Ford. Just seemed to be the best deal for the money to me.