Well the 100-400 is certainly going to give you a lot more scope than the 70-200. Put it this way ... there will be many more occasions when you'll wish you had the extra reach than there will be when you wish you had the missing 30mm at the low end!
But if you're really worried about that missing 30mm, you could consider the Canon 24-85mm which narrows the gap a bit. I'm trialling one at the moment as a possible "walk around" lens, and am pretty pleased with the results. Its not L glass, but it is USM
and seems to offer a consistent performance across the range.
Before anyone jumps in and tells me to stick with "L" - size and weight are a critical factor for my use of this lens ... also, I wanted something that worked with the camera's built in flash.
Colin K. Work, Pixstel