In my opinion
[hr]
if you're not planning on a live sand bed (lsb), I recommend a thin layer of sand. Its easier to keep clean, in my experience. However, if you have fish like a rainsford goby, or a sand muncher, like a yellow headed goby, you may want a level that keeps him/her comfortable. You can keep an area of the tank that is deeper than the other for fish and inverts that like the sand, but I like a very thin layer, almost where at times the glass shows through. The reason is I found that a thick layer of sand, 2 or more inches, seems to require a good amount of stirring in order to prevent stagnant areas where cyno bacteria and other unwanted algae like to form. It might look nice in the beginning, but unless you have good circulation over and inverts keeping it stirred up, it will quickly get unsightly and require you to stir it up and keep it clean.
There will be others that will have different opinions on this, sometimes you just need to experiment to see what you like.
If you are doing a live sand bed, a natural filter when its done right, then research that in other areas on this web site. There are plenty of threads.....