Upload & Sell: On
I use them both extensively. I read all the different threads here evaluating the 70-200II with a 2xTC, and I concluded that the image quality was probably on a par with the 100-400, but the AF in AI Servo was going to be slower than the 100-400. Since I already had both, I just kept them. If I didn't own the 100-400 yet, I would have been tempted to pick up a 2x TC and see if I could live without the 100-400.
Here's what I use the two lenses for the most:
100-400: Soccer, baseball on full-size fields, nature (mostly birds), air shows
70-200: Ice hockey (all winter at f/2.8), sports car racing, low-light events (weddings, christenings, etc), some nature, used to shoot Little League where I used the 70-200 more than the 100-400.