DSB and Plenum systems are different types of filtration. They incorporate the same idea with the use of 4-5" sandbed. The DSB cannot have sifters in the sand because they can get to the bottom of the tank and disturb the oxygen-free layers. This is very bad for the effectiveness of the sandbed. The Plenum system has a screened off area for the same oxygen-free layer. Sifters cannot get to this layer, so they are safe to have with a plenum system.
Now, I don't want to confuse you any more than you are, but, cukes are not considered sifters. Gobies and sand-sifting stars are. Most of what is eaten in the sand quickly reproduces, but gobies deplete this faster than it can multiply. Plus gobies spray the sand with no regard to where the sand falls-landing on your corals and irritating them, possibly damaging them. That being said, the DSB, IMO, is better for than reason alone. I know others will debate this until we are blue in the face. So, read about both systems, then make up your own mind. I nope I cleared it up a little bit. --Bob