Between an ophthalmologist and neurologist. Which one would be better to see if fast failing vision?

Ophthalmologist. See an ophthalmologist because it most likely is an eye problem.
An ophthalmologist. You need to see an ophthalmologist for rapidly decreasing visual acuity. If it is thought that the problem is neurologic in neature, you will be referred to a neuro-ophthalmologist and later on, to a neurologist if needs be. Always see a vision specialist first when dealing with vision issues.