Yeah, I used to be a big fan of McFarlane's art on Hulk and ASM, but looking back on it, I don't like it much at all. His humans--to me--often resemble flashy, hyper-detailed wads of putty.
On the flipside, I was never big into Mignola's work back then, but now, I'm a big fan of his very distinctive style.
Besides stating my opinion, of course, I'm not sure how anyone can quantify one artist's pencilling ability over the other... I imagine both McFarlane and Mignola could approximate each other's styles with a gun to each of their heads. I don't think the perceived simplicity in Mignola's style should suggest that he is unable to do more busy, detailed sorts of drawings like McFarlane, or that that sort of detailing equals greater skill. That being said, I find the distinct style of Mignola's compositions much more aesthetically pleasing.
Out of curiosity, would someone mind posting or referring to a good fight sequence drawn by McFarlane?