I've bounced back and forth between aluminum and carbon stems on my new bike. I have a couple with my stash. Not much difference. I got an aluminum stem to replace the carbon one that came with my bike (it was too long). The aluminum stem is lighter by ten grams, or something like that (which is nothing). I do use a carbon seatpost with my carbon bike, and it's a one bolt, I can get it dialed in where I want it. I have an aluminum seatpost on my old steel bike. Carbon has a nice look. I like to look down and see that zoot carbon stem, but the aluminum one works very well. Some of the prices for the carbon stems and seatposts are outrageous, when an aluminum one will work just as well.
If you want a nice looking aluminum stem or seatpost, look into a Ritchey WCS 4-Axis wetblack..