I have had both in my large (3000+ gallon) coral holding system. The plenum was up for about a year and a half. When we moved our facility I went with deep sand bed because I didn't want to build plenum racks. That system has been going for about 2 and a half years. I never had any problems with either and have yet to see a difference between the two.
If you do a search on this board for 'plenum' and 'deep sand bed' or 'DSB' you should pick up a lot of threads on this. My opinion is that a DSB provides the advantages of a plenum without the added difficulties.
