If you are unsure why dont you do what we all do.. start with a shallow bed, say 2cm and take it from there, you can always syphon bits out slowly if you find that the system is not performing after a year or 2 once the tank has semi matured and as Mofro says, as you mature in reefing, then make the decision. If you don't go sand bed now you will eventually IMO.
There are so many different points of view on this subject and it is all subjective as all our tanks run in their own unique way depending on what you have in the tank, light cycles, additives, husbandary, feeding habits, equipment etc etc the list is so long, asking advice only equips you with information to work with... now you have to decide what will work for you, my point of view may not work in your system, IMHO.