Which one is the better one - the normal core or the essence you make via combo?

The core is a nice light blocker if you don't have something like the dark goddess rod instead. However, the essence is the best dark blocker you can get and it also has the potential to block light as well as dark. The essence also has a 10% heal on it, but it's a T11 weapon. I would say the essence is definitely better but you could use the core until your pet is T11 and then make the essence.