They offer different treatments and different benefits. I've talked to most Dermabrasion companies and looked into most machines. I have decided that I will be going for the Skin For Life machine that offers Crystal and Organic Walnut, along with LED and Oxygen treatments.
Talk to as many companies as you can to find out which you think will work best for you.
Diamond is great, offers a lot of work possibilities. However talking to companies I don't think it is as flexible a treatment as crystal based. At the Olympia show I had a Diamond Dermabrasion where they just drag the diamond head over the skin and it has a suction to remove the loosed skin cells. Not that great and experience ... I didn't rate the machine.
There is definitely a good place for diamond - around the eyes it can be a bit more gentle maybe and of course no worry about loose particles falling into the eyes. Some people do prefer the feel of diamond. You just need to look at what you want to offer your clients.
One option is to go for something like the Carlton machine that offers both Crystal and Diamond. It's a great machine and one that I was thinking of buying before seeing the Skin For Life one.