Yes, though I'm used to Canons, I wouldn't complain if I had a Nikon. It just annoys me how the controls are reversed (well at least for a Canon user).
However, I wouldn't give up my Mark II for anything in the world. Combine that with my 50mm f1.4 (oldie but a goodie, and I honestly can't tell the difference between this and the f1.2), 16-35mm wide angle (everyone needs at least one wide angle), and 70-200mm f2.8 white lens (you haven't tried a real zoom lens until you've picked up one of these), and you have one of the best sets a photographer can ask for. Honestly though, the 50mm is the cheapest of them all, and is the one that is on the body 85% of the time.
Oh yeah, to actually answer the OP's question:
While I have never used the Sigma lens that you are talking about, they make some fine lenses. They don't compare to the equivalent "name brand" when it comes to speed + auto focus quietness, but that doesn't matter when your taking pictures of teeth. Since they offer similar clarity and sharpness, you might as well save yourself the $$$.
BTW, wouldn't a flash blind the poor sap your taking the picture of? I'm assuming that you'd have the camera pretty close to his face.
