What is the big differences between a drum sander and a thickness planer? The only thing I know is that the drum sander does not remove much material, but can do more area at a time. But the planer can remove a lot of material but lots of money would have to be invested to get a 24 inch. What are other big differences with them and Pros and cons for each other. That would be extremely useful!
You're correct that the planer can remove much more material at a time. With the planer, it is also simple to adjust the depth for removing more material in a pass. With the drum sander, you can change the grit of the paper to take off more material (nowhere near as much as the planer) but that is a more painstaking process.
That said, I own a drum sander but no planer. I used to make acoustic guitars and work with figured wood. In this case the planer is of little use since it will take out chips where the grain angles up. The drum sander does not chip-out the stock and allows smooth and uniform surfacing of wood down to 2 mm thickness, even thinner if required.
So, preference rather depends on what you're doing. I've also used my drum sander to fashion oak window jambs and trim. It required more passes than a planer would have but I would have wanted the drumb sander for finishing anyway, I was happy with my choice of tools.
I own both. The comments made so far seem right on from my experience. The one major advantage of a drum sander is: because of the open end, it can surface a workpiece that is about twice as wide as the planer. Just rotate the piece 180 degrees and send it through again.
If your wood contains resin, you may face two problems: 1) the resin can clog the sandpaper (use saw blade resin cleaner to clean it), and 2) resin buildup on the paper can actually ignite burning a "nice" black line on your workpiece. The dust can ignite also. In general a drum sander is trickier to use and requires patience.
My wood (Guanacaste) is very susceptible to tear out and even the sander will leave many tiny tear outs. Fortunately, the flaps lay down during finishing.