Which one is better?
No one.
The scanline renderer is WAY faster, but lacks the reflection/refraction qualities of a raytracer.
The raytracer is very slow, but has a better quality for metal, glass, and water(!) materials.
The first is better for animations, the second for stills (unless you have some spare years to render...)
Some Renderer like RayMAX combine both to give you SPEED and QUALITY in one engine. You select which objects have to be raytraced, and if the ray meets such an object, it is traced accordingly, but for all other objects the normal way of calculation is used.
(I don't mention Lightwave here because it is a polygon-based renderer, not a scanline renderer, although it has selective raytracing, too.)