It depends on cover and distance involved. Close up, and he is armed, you will not have time for either, you may have to take him on hand to hand. Medium range with cover, use backup if you have one, then with the security of a backup available, then clear your main if you can. No cover, do which one is faster for YOU!
If you are a street cop, ALWAYS carry a backup, as you go to trouble, civilians should using their guns to get away from trouble!!! A major difference in tactics!!!
All this depends on your skill, training, and instincts ... there is no right answer as combat is chaos, and chaos is random non-solutions waiting for order to manifest out of the madness!