DMT:
Stay flat forever; no need to level them. Cut any kind of steel and does it well. No wasted effort. Grit or mesh ratings are more coarse than they seem. "Extra fine" (9 micron) produces a finish that's more coarse than a Spyderco medium.
Can be splashed with water, used with mineral oil, or used dry.
Good feedback, but can take a light touch to remove the final burr. Speaking of light touch, you DO NOT WANT to use much pressure with DMT stones. If you do, you will rip the diamonds out of the nickel plate making your plates much less useful. Ask me how I know.

The XXC plate is much tougher and you can use a good amount of force on it without damage. If you are doing heavy grinding at all (or do not have the patience to spend a lot of time setting initial edges) I highly recommend the XXC.
Waterstones:
Many require soaking in water. Others only require "splashes" of water to work. All need water. No oil allowed! Oil will ruin the stones.
Will dish with use and need to be flattened. Typical practice is to flatten them before every session. This varies from stone to stone as some are very hard, some are very soft, and some are in between.
As with hardness, the grit rating and "feel" of a stone varies wildly from brand to brand and model to model. You need to study this a bit to get what you want. Many people end up with a wide variety of water stones because they are all so different in many ways. Some people even collect them.
Waterstones will cut basic steels with ease and most entry level "super steels" won't be a problem either. Anything with a lot of Vanadium will be a problem for most waterstones. S90V and S110V probably won't work on the vast majority of waterstones.
The feedback from these also varies a lot. They are generally pretty soft, which means less feedback, but there are harder stones too. I have trouble with my small set of waterstones because of this. However, the slurry that forms (or something else about the friability of waterstones) makes deburring easier. It also seems to discourage big burrs because of these properties. Dealing with burrs is easier on waterstones I think.
I think those are the major plus and minus points for these two stone types. Good luck on your decision.
Brian.