Pepper mace works great, in my experience.
Do not use the kind that shoots a mist-get the stream-type. The mist is too likely to blow back on you.
It the dog is really agressive, don't bother with the Mace. Use the gun. It can take too long to spray mace, see if it worked, then pull your gun, get on target, and shoot. A dog can cover a lot of ground in that short amount of time. So can a human, for that matter.
I suppose you can hold the gun in your dominant hand, and the mace in the non-dominant hand, and use as necessary, but that's something not to be attempted without a lot of practice.
Getting a good hit or two with the gun (under stress ), while a fast, dangerous animal is coming at you from a low angle is not a piece of cake.