Depends on the tooling you're going for. Synthetic diamonds are great for creating abrasives, which makes up the majority of industrial diamond tools.

But for a lot of machining where you need a large cutting head, industrial diamonds are a little too small. I think most synthetic diamonds are less than a millimeter in diameter. They can be made larger, but they take a long time and aren't very cost effective.

The vast majority of diamonds mined are industrial grade, especially the larger ones. So you end up using a bit of both depending on your application.