The main benefit of a plenum (and I'm not stating this to start another debate about DSB's vs. Plenums) is that a plenum system requires less sand and the grid that seperates the lower layer from the rest (if correctly installed) keeps the lower layer from being disturbed.
A deep sand bed will do the same thing as a plenum, however with a DSB you need at least 1 to 2 more inches of sand (more cost, especially in a BIG tank) and there is a slight risk of sand creatures disturbing the lower layer of sand (which you want to avoid) and the plenum, when correctly installed, will prevent that.
They BOTH are great and I would go with either one, rather than neither, as your tank should be more self-sustaining and healthy with either of them.
[ April 19, 2001: Message edited by: Sundog ]