new 20g and can't decide

[Richer]

The whole point of a BB system is that there is no sand for detritus to get caught in. So, in short, adding sand into your system afterwards would defeat the purpose of a BB. Unless of course, you add it in after you decide that you definately do not like the look of a BB system, or you've decided to keep sand dwelling organisms.

As for the main topic... I'd say it depends on what you want to keep. Obviously, if you want to keep sand dwellers you'd want to have a sandbed. However, IMHO, if you do not plan on keeping sand dwellers, I would just go with a BB system. Advantages include easier cleaning of the tank, the possibility to have a high turnover rate in your tank, etc. My 30g half cube is a BB tank, and I love it. I have a nice turnover rate of about 70-80x (mag9 return, 1xseio 620, 1xseio 820, 1xMJ1200) and I don't have to worry about kicking up a sandstorm, plus the flow is more evenly distributed around the tank... rather than concentrated in the top portion of the tank, so I don't have to worry about blowing the crap out of my corals. The fish also seem to enjoy the high rate of flow as well. A lot of the detritus that is produced in my tank gets kicked up and sucked into my skimmer.

-Rich