I have all 3 types and find the scissor type to work the best with my girls. I think it comes down to the quality between the scissor type and the guillotine. I have had crappy ones of both and good ones of both. I like the grinder but it takes a bit longer to get it done and with a freaked out chi it can be VERY difficult to get done. The grinder is really nice for a dog that will let you use it for the same reasons that Brodysmom mentioned.
I use a grinder too. Love it. No more quicked nails!!! With one of my chi's, I even pay the groomer to use a grinder. (she just will not stay still with me, and starts to get aggressive after a while, nibbling and snarly face) Sue
I use the scissor type, then finish off with a nail file, but a grinder is the best (mines currently broken) - They take a little getting used to for both owner and dog, but so much easier and better than normal nail clippers!