Hi ..it is really down to personal preference and what you find easiest to use.Some people start in nurbs and then convert to poly, others start straight in a poly and some only use nurbs.
you can even use a polyproxy object if you like.
As for which primitive to start with..well it is a matter of judgement, obviously a cube would be more suited to straight boxy type parts and a cylinder lends itself to more rounded objects.
nurbs are more suited to curvey smooth objects such as cars for the simple reason that they are constructed from a curve and give a smooth rounded result, but it doesnt mean you can't make the same thing from a polygon it would just be a different method. ploygons are more suited to hard flat type objects but again it is not impossible to use nurbs.
I suppose there is no right or wrong it is just what you find most comfortable to work with.
The only thing to bear in mind is to keep it as simple as possible as you start and try not to get to dense a mesh..you can then add detail once you have the model roughed out to shape and size.