Well, my tank had been bare bottom for about 3 years. Up until about 2-3 days ago when I found a bit of gravel in my shed, washed it and put it in. It turned the tank a bit cloudy, but that cleared after a while. It also depends on whether or not you have bottom feeders in your tank. If you have fish with sensitive mouths then the generally prefer sand, such as corydoras. However some species can be happy with gravel, such as plecos and cories, which can live on both happily. Make sure the sand is not too sharp otherwise it can harm the barbels on some fish.
Now, it also depends what kind of plants that you want to keep. If you want to keep live plants, it's better to have a fertiliser substrate which is then covered over by gravel or sand. Of course that is not necessary but certainly won't do the plants any harm.... I prefer the look of dark sand with lots of lush, green plants. I would get sand, but the dirt shows up far too easily for my liking.
For bare bottom, it shows where you need to clean, but you can't bury plants in anything. So, personally, I would rather have a substrate. Others will probably have different opinions though.