I was slamming a D-ball this morning and watching the filler puff out of the stopper.
A Dynamax ball is much more forgiving on your face if you miss a wall ball rebound than a D-ball.
For a slammer I'd recommend making your own out of a basketball (much cheaper than a D-ball if you don't count time invested).
Pierre Auge had a nice write up on how to make one. I think I posted my How-To on making a wall ball over on the Xfit boards under equipment that was based on Pierre's instructions.
For weight variation you can fill them with things other than sand. Bird seed for lighter. Steel or lead shot for heavier (lead is cheaper as I recall).
Sand can actually be a pain if it isn't completely dry.