In my case, it seems to depend on the terrain. If it's very flat and level, I figure the cruise control is probably more constant than my foot. However, my route to/from work has a few "dips" (both in the road surface and otherwise 8)), and I find that by keeping my foot steady and letting the car just slow down/speed up by 1mph or so gets me better mpg.
I just got home from a long road trip to S.F. and back to L.A. with lots of driving in between. It did seem to me that I was able to get better MPG using my foot than I was getting with cruise control. With cruise on, it felt like the car was constantly using the gas engine even on downhills, like it was using engine braking to maintain my speed.