I kept worrying about gaps in focal length when I was looking into my L lenses but from my own experience I wouldn't worry about it too much - usually you can either move closer/further away or crop later. I think the 100-400 sounds like a better lens for what you want.


I would suggest though that you consider the 24-70 or 24-105. I have the former and when I used it on my old 400D (before I got my 5DII) I rarely found that the 24 end on the crop sensor wasn't wide enough. But then again I rarely do landscapes.


If you're unsure, try paying close attention to where you're placing your current zoom lens, and try using it without going wider than 24mm. Also try shooting without using the 55-100mm range. That should help you decide which compromise you're willing to make.