Agree that for compacts and ultrazoom fixed lens cameras, there is no significant difference. In fact, you can see the stabilized effect on the screen or electronic viewfinder no matter which stabilization you use for the compacts. For D-SLR, which one is better depends on your needs.
Optical lens-based stabilization has its advantages, being able to see the stabilized effect, theoretically better AF performance in low light, and able to customize type of stabilization for the lens. But these are not huge advantages. Being able to see stabilized effect is useful but not a deal breaker, you still have to check your images on the screen for sharpness no matter which type of stabilization you use. For the newer cameras with electronic viewfinder, you can actually see the stabilized effect in a camera with built-in stabilization.
However, a bigger reason to favor lens based stabilization is that the in-camera stabilization system may have lower AF performance in lower lighting. This is because only the main imaging sensor is moved whereas the autofocus sensor is not moved for shift correction. And for my Canon EF 70-200mm f/2.8L IS, I can switch the mode of stabilization when I do action panning, to minimize unwanted lateral/horizontal correction during my panning. Canon also has a new hybrid stabilization for its 100mm f/2.8 IS macro lens.
But on the other hand, many people will like to have image stabilization for all their lenses from past, present and future. This can cut cost and also gain image stabilization for every lens you own (that is compatible with the camera). So this can be a significant cost advantage.
So if money/budget is not a significant factor, and you want a slight advantage in handheld performance with the best lens, then Canon and Nikon lens stabilization system do perform better than in-camera stabilization. But bear in mind that neither stabilization system can match a tripod and good techniques. And also remember that image stabilization only corrects camera shakes, will not improve subject motion blur. So getting a fast lens with wider aperture is preferred over a slow lens with IS (though having a fast lens with IS can be pretty sweet but will cost you). If you want the sharpest photo, a cheap tripod will trump any stabilization system any day. For most people, neither stabilization system will make a big difference in their image quality.
In terms of how this will affect which D-SLR system you pick, the built-in camera mechanical image stabilization systems like Sony and others can give you significant cost savings if you plan on buying a lot of lenses (especially third party cheaper lenses without IS). But you won't see a big saving if you only plan to buy 1 or 2 manufacturer lenses. However, if you like to have the most extensive choice of lenses and accessories, the most compatible system with third party softwares (eg. tethered shooting, etc), then Canon and Nikon will fit you more. This is not to say that Sony, Pentax, Olympus and others don't have quality equipment or lenses. They do, but their selections is not as extensive as Canon or Nikon. And third party softwares and special accessories are not always available for non-Canon/Nikon brands. These are the more valid reasons for those who pick Canon and Nikon. The decision is not usually based on which stabilization system a particular manufacturer uses. As mentioned above, the image stabilization choice has more to do with building the D-SLR system and its loyal clients' preference. Canon and Nikon both have already had their lens stabilization system well established before Minolta push out its built-in camera mechanical sensor shift stabilization system in its Maxxum D-SLRs. If Canon and Nikon shift to mechanical stabilization, then can you imagine how furious those clients who have spent tens of thousands of dollars for those IS lenses?