Choosing a telephoto zoom lens can be quite easy, once you know what your needs are.
Since the Fuji s5 pro that I own uses the Nikon F mount, there are many good lenses to choose from.
Sigma, Tamron, Tokina and of course Nikon all produce lenses ranging from economical, entry level glass up to and including professional grade, open aperture lenses.
Due to the fact that I chose a 17-70mm lens as my "normal" zoom lens, a 70-300mm zoom lens made a lot of sense for my telephoto glass.
Even though the high end, very fast, telephoto lens offer the best image quality their weight and price tags are a bit more than I desired.
Both Sigma and Tamron offer 70-300mm lenses at very economical prices. These could have been an option for me but both lack the image stabilization feature. This in its self is not a big deal, except that being able to use the lens with out always having to carry a tripod was an option that I felt was an important one.
The Nikon 70-300mm f/4.5-5.6 ED G VR became the obvious choice.
This is a fantastic lens. It has good build quality but does not have the size or weight as to make it a burden to carry it around.
The image quality is wonderful, very sharp images with good corners are the result.
The image stabilization functions well, allowing the lens to be hand held quite easily, even when shot at full zoom and fairly slow shutter speeds.
The lens also perfectly compliments the Sigma 17-70mm that I use as my normal zoom lens.
All in all, a very useful combination.