I am a believer in variety so I would say "all the above". My ornates eat slugs, red wrigglers and nightcrawlers very well. Of the three, I like the red wrigglers best. Red wrigglers are the smaller bright red worms you see commonly sold as fishing worms. Nightcrawlers are the liver colored ones you find stored refrigerated and are quite a bit larger. Slugs are... well you know.
One benefit of slugs is they are free around my house. However, if you use commercial pesticides or fertilizers, the critters around the house could hurt your turts so might want to consider buying commercially raised.
Another good protein based food for turts is crickets. Mine really go for them and I think having to chase prey is very healthy for pets in captivity. The local pet stores usually sell bulk crickets cheap in a variety of sizes.