What is your primary photography interest? Knowing this key question would probably alter my responses below, so I'll try to keep focused on focal length. Aperture will change based on your needs (wedding/event pro would need f/2.8 glass, for example).
24-105
16-35 f/4
70-200 f/4 IS
Light weight versatility that comes in under the cost of the 70-200 f/2.8 II. If you need low light shooting, either add primes in the desired focal length, a flash, or tripod. F/2.8 zooms won't get you that (really) low light capability vs. the f/4, so save yourself the money.
gonzogolf wrote in post #17876983
What do you want to do with the longer lens, be specific. The 100-400 is a great wildlife lens, but not the best option for all around use. The 70-200 is more of an all arounder but short for sports or wildlife. The 2x isnt a good option or substitute for the longer zoom.
This echos what I've experienced. I've had the 70-200 f/2.8 II for years now and just recently added the 100-400. What I've found is that the 100-400 is AMAZING for wild life shooting in good light, but outside of that its focal length and aperture is more limiting for all around use. Basically, I have to make it my priority to shoot longer, and it changes how you do photography vs. just picking up a general purpose focal length lens and going to town. It's the same with ultra wide (aka 16-35), I have to change my perspective and vision in order to make full use of what it brings to the table. Again, depending on what you want to shoot, this may not mean anything because some people are ultra-wide wizards, or maybe they live at >300mm (check out the 100-400 II review thread!).
So, while the extender route may not be the best choice, it would be the cheaper route, and that would allow you to see if having that 400mm reach is right for you. I'm actually having the same debate right now, but instead went the zoom-first route
For me, outside of shooting birds and wildlife, I honestly can't see myself using the 100-400 all that much, whereas my 70-200 gets used rather frequently for portraits and the occasional event. In this case, this is the one f/2.8 exception I make for my collection; and you wouldn't think that 30mm of range (70 vs 100) makes a difference, but for some reason it does!