Advantages to Reader vs. Preview: Handles forms much better, browser plugin for in-browser-window PDF viewing, supports some fancier PDF features that Preview doesn't, WAY more settings. Supports digital signatures. Seems to render some complex PDFs faster than Preivew.
Disadvantages: Much more memory-hungry, launches significantly slower. If you don't want or need any of the fancy features, the simplicity can also be a definite advantage. It can also, in some cases, save a copy of a filled-out PDF form that Adobe Reader won't (Adobe wants you to buy the full version to save filled-out forms).
There's nothing wrong with either as far as I'm concerned, it just depends on what you want. If you just want to see a product manual now and then, Preview will probably handle it better and more efficiently--launches fast, not hefty on the memory requirements.
If you like the extra features of Adobe Reader, want the browser integration (though I think you can use that without having Reader be the default viewer), or prefer the interface, Reader has the edge.
It's free and doesn't install anything low-level other than the browser plug-in (which you can remove if you don't like), so I'd say try both, set whichever you prefer as the default for PDFs.