Zircon, without a doubt. Although I have upgraded to the super-cool top o' the line model with built in display and coffee maker, every time I cannot find it, I reach for my older, stripped-down model and am amazed at how great it works (which is why I've never parted with it).
The new one shows you where the center
of the stud is, not the edges. Being the skeptic that I am, I tried my friend's out before buying my own. I actually screwed a scrap 2x4 to some scrap sheet rock and to my suprise, it was dead nuts accurate.
My house is vintage 1911 and the Zircon behaves quite well on plaster walls, even with some minor undulations. In fact, my attic is filled with vermiculite (don't go there) and believe it or not, the Zircon is capable of distinguishing the difference in density between the ceiling joists and insulation, even through plaster! Granted its operation in this instance is somewhat better than marginal but under the circumstances, that's pretty good!
So all in all, I really cant stand Zircon products.