I haven't noticed a significant difference in AF between the two and my keeper rate is much higher with the 70-200mm. I haven't actually checked the weight difference, but my shoulders say the 70-200mm is lighter than the 100-400mm, even with an extender (that might be all in my head though, I need to weight them both). Plus, take the extender off and you have a great walk around lens for static shots - even in dimly lit hangers.

When I'm thinking about a new body or lens I'll usually rent them on a few occasions just to get a feel of how they do in different situations. Might be something you want to look into before laying down the big bucks.

Additional note on the Tamron 150-600mm: I found the stabilization to be horrendous. After the first dozen shots I turned it off and have never turned it back on.