When going for extra reach you need to be sure that extra focal length translates into extra detail resolution. A good 50mm lens can produce more detail resolution than a not so good 100mm.
For example, I have a 500mm f8 mirror lens, and a 70-300mm lens which many people assured me would give me obviously better detail resolution than such a rubbish 500mm mirror lens, mirror lenses being "well known" to be rubbish. In fact it obviously doesn't. The 500mm lens very obviously provides a lot more detail resolution than the 70-300mm at 300mm. On the other hand I've also compared the 500mm with a very expensive highly regarded 70-400mm f4 lens. At 400mm that zoom does produce slightly more detail in dim areas of the image than my lower contrast 500mm mirror lens. But only the kind of extra detail that an experienced pixel peeping photographer would be able to notice, the kind of extra detail that you might just be able to notice if you used a magnifying glass on a good A3 print.
On a dim misty day a 100mm lens can produce just as much detail of a distant big ship on the horizon as a top 600mm prime.