The ham is from the rear leg, the butt's from the front. they are both great for sausage making.
The important part is that you add the fat that the recipe calls for, since the ham is about 15% fat and the butt is about 30%. If your recipe calls for just butt then you'll want to add some of the hams back fat to make up the difference.
Thanks for the feedback. Here is a picture of the ham chopped up into chunks... there was a good layer of fat under the skin and I tried to keep on as much as possible... maybe I'll throw in a bit more fat just to be sure.