Long lens...

[lmontes1974]

If you need only one: 200-600 f 5.6-6.3 or 100-400 f 4.5 - 5.6?

[TrekRover]

For me personally, it was the 200-600, for the maximum focal length reach. The reason and way I built my gear line up was due to what I had first and to cover all focal lengths without overlap. I started with 70-200, then 24-70, then 200-600, then 12-24mm to essentially cover the spectrum of 12-600mm adding on the 2.0x TC I can cover up to 1200mm add on crop I think ~1600mm

[lmontes1974]

and in terms of quality image?

[TrekRover]

I've never used the 100-400 so I can't offer a comparison, but the 200-600 is fast and it has pretty good image quality. Here are some sample images.

[jb]

The 100-400 and 200-600 are very different lenses in use.  So it really depends on what subjects you're shooting and how you expect to carry either.  The 100-400 carries exactly like a 70-200.  Same size and weight.  Fits in the same bags.  Easily shoulder carried.  The 200-600 is very different to carry and use.  It's size is more akin to a supertele like the 600 f4 due to the constant length.  You need a much longer bag to carry the 200-600.  Hiking or simply walking around with it requires a different approach.  It's nearly a kilo heavier than the 100-400.  I can hand hold the 100-400 all day (and I do).  I want a monopod when using the 200-600.