Generally speaking, a drum is used for stock removal and a platen is used for finish sanding; however, drums are also used for finish sanding in some applications. A rule of thumb would be, if you need to remove more than 0.003 to 0.004 inch, you should use a drum, otherwise a platen may be used. The difference between the two is also seen in the finish. A drum will produce a short scratch pattern, but it is deeper on a given grit. A platen will produce a longer scratch that is not as deep. You really need to determine stock removal requirements and desired finish to decide which will fit your needs better.