To plenum or not to plenum, that is the question.  The answer is... I don't know.  I had originally planned for one, originally set one up.  Then I noticed that parts, rather large ones, of the sand bed were turning as hard as concrete.  All clumped together.  Now truth be told, I do not know if that was caused by it's nature or by my lack of skill with additives.  But my guess is probably the latter.  I took it out none the less.  That was a pain, let me tell you.  So if your going to go with one, try to investigate what your doing it for and what your getting yourself into first.

    I will say that they were devised for purely scientific reasons.  Not for the average reef tank, but actual research of organisms.  They are primarily meant for low bio load systems, yet some still feel they can be pushed to the limit.  They usually end up with a form of DSB with an open area at the bottom, not the pristine tanks of their originator.  I think that this issue is what caused many to just rip out the plenum and use just a sand bed.  I have seen some articles that actually say that one is better than the other.  In my opinion, they each have their place and do what they do best.  It's up to the individual aquarist to decide which is best for the application they are using.

    So, after the plenum, I switched to a DSB of sorts, which was really just the same sand I had over top of the plenum dumped on the bottom.  Possibly I didn't seed with enough critters after setting it, or just had the wrong something or other but that didn't go well either.  Seemed to me to be more of a nutrient sink than anything helpful.  My opinion after reading the material available to me on that topic is that it generally isn't for beginners.  The main reason I say that is that I do not feel that I had a clue how complex they are when I started out.  You can't just dump sand in and call it a DSB, it takes considerably more than that.  At the same time I feel many understress the need for seeding one properly and then explaining the needed maintenance well enough for the average beginner.  Seems to statistically have about as much success as a plenum, maybe more but not much.  Looks good on paper though.  I just don't know that the potential benefit is worth the pain in the rear, or the pocket book to keep it working correctly.