Your brain, and all your other cells, can use either glucose or ketones for fuel. They prefer glucose because it is more efficient, but there is no measurable cognitive impairment in those on keto diets - in fact, ketosis has been shown to improve cognitive function in Alzheimer's patients: study.
In general, this isn't an either-or thing. Your body is doing all of these pathways all the time, just the relative degree of them can be shifted depending on what's available in the diet.