No question the 2.8 performs better in low light. It just comes at a fair price both financially and in weight. The next generation IS helps bridge the normal disparity between the f4 and f2.8 IS. I see the 2.8 IS as a portrait lens. Consider the 100-400L. It will give you the reach you want for wildlife if that's what you are talking about at dusk and dawn. If you want a fast lens for the price difference between the two 70-200's you could get a prime. Both 70-200's take the 1.4x TC well so you can squeeze 280mm out of them but you loose one f stop.