Google three terms: Capture sharpening, creative sharpening, and output sharpening and you will see that thousands of electrons have died on this subject. Here is a good, basic read:
http://www.bythom.com/sharpening.htm
Basically:
-- Capture sharpening, as in Raw Fine Tuning is designed to compensate for the intentional blurring of digital images by the anti-alias filter. Typically, the defaults are fine and even if you slam the sliders, the effects are subtle.
-- Creative sharpening is what most of us do. As noted above, digital images are, by default, blurred, so you must do some local contrast adjustment to make the edges stand out. Apple's first sharpening tool, called "Sharpening" was fairly basic and generally reviled. The second version, called "Edge Sharpening” is an absolutely state of the art, multi-pass sharpening tool. The general rule is that you sharpen “busy” images (wheat field, mountain range) more and smooth images, like a woman’s portrait, less. (There is a whole art form of sharpening edges and blurring skin in portraiture.)
-- Both Contrast and Definition have a sharpening effect; the first blunt and the second quite precise. (Definition is actually a form of low level, extremely wide radius sharpening - see the end of Thom's article.) I prefer to use them first.
-- If you look at the bottom of the article, you see see a section on selective sharpening. Happily, Aperture 3.0 will allow you to do that with a brush; no Photoshop masks and layers need apply. (Nik's dFine also lets you do this with their U-Point section method, but that is another discussion.)
-- The final step is output sharpening, based on the idea that you need different levels of sharpening for print (more, as ink spreads) or screen or web, depending on the size of the image. Nik’s dFine has presets; the rest of us use the Mark One Eyeball. Aperture offers additional sharpening in the print dialog, but not in the export dialog.
Koen’s suggestion of mixing and matching both Aperture sharpening modules is most interesting. As you can see from the Thom Hogan article, Aperture offers a wide range of state of the art sharpening tools without all of the pain and agony of doing this all by hand in Photoshop. You should have no problem oversharpening your images to death. (That's where a non-destructive workflow is soooooo nice! 😉
Final Note: Sharpening requires Aperture to examine almost every single pixel on the fly. You will get better response if you turn sharpening off until you are happy with all of your other adjustments, otherwise, you may get noticable lags as the image is redrawn and then resharpened after each adjustment. Obviously, this gets worse at full screen and with big TIFF images.
Hope this is helpful.
