I have not even considered the weight issue here... As you say, the 100-400 is pretty light, and it actually feels lighter than it looks, right? Well, after a few weeks of this lens permanently attached you sort of forget about the weight, that can be painful if you're holding the camera for extensive periods of time. After some time I put on my trusty 50mm 1.8 and as soon as I picked up the camera I thought I forgot to load both batteries (using grip), yeah, it felt like featherlight. And now, considering that the 70-200IS weighs 1600g, Sigma 120-300 2600g... the 70-200 wins the round (100-400 weighs 1360g, ). Despite owning both, a tripod and monopod, I'm not really a big fan of using either, I just like to be unobtrusive, flexible, easy to move, and unnoticable - if that was even possible with a huge camera and a grand white lens.
Long range is always useful, as you know, there are plenty of times when you wish you had a 1200mm lens, right? So why would I be willing to trade 400mm for 200mm? Sharpness. That's it. I've taken some shots @ 400mm and they turned unsharp and I was so pi$$ed off and I just couldn't believe it. Still, those extra 100mm are very, very tempting....
Well, if it's not obvious, I'll state it for the record: I do love the 100-400 but it really requires sunny days and I'm not living in California, if you catch my drift. F/8 is when it becomes really sharp and f/8 I can usually not afford.
My lens are as follows:
Tamron 17-35 f/2.8
Tamron 28-75 f/2.8
Canon 50mm f/1.8 II
Canon 50mm f/1 (friend let me have it for a few weeks, btw, he's selling it, anyone wanna buy?
)
Canon 100-400L
What I need the lens for: I've tried birds, but they take time and patience and I run out of both quickly, motorsports, airshows, portraits, candids, concerts (I have seen guys with 70-200 2.8IS there), and last but definetely not least - to impress chicks. 
5D and holy trinity of primes. Now the 90mm TS-E TS-E fly bit me. I hate these forums.