I'm debating if I should get the 70-200mm F/2.8L IS or the Canon EF 100-400mm f/4.5-5.6 L IS USM. I would get the faster 70-200 if i go with that, but i don't know if the extra 200mm is going to be needed. I do take a lot of nature and sunsets, but should I go with the 70-200 and go with the 300mm 4.0L later.