Supposedly friction shifters work better as the number of gears increase, not sure if that's true or not though.
I prefer friction, on my touring bike, but I would say that indexed works better as the number of gears increase, as in more cogs on the cassette. As you get more cogs on the cassette. like a ten speed cassette, it takes more effort to fine tune with friction. I use friction with a 9 speed, and I love it. The added benefit of friction is that it is easy to run from the small cog to the top, or top to bottom in one fell swoop.