Greg OH wrote:

If you're like me, you've spent some time considering your telephoto options. First of all, 200mm isn't long enough for me. So that left the 70-300 and 100-400.

70-300 advantages:

70-100 range is something I use a lot. But, I already have two lenses in that range (24-105 and 85/1.8).

Maybe better IS. 6 stops instead of 4. Not really a big deal since 4 stops gets you 1/25 at 400mm using the 1/FL rule.

Slightly larger aperture.

100-400 advantages:

300-400 range. I don't have anything in that range, and nothing else is even offered. And, the matching TC make 600 or even 800mm possible.

Value. In most systems, the 100-400 is usually about twice the price of the 70-300. Here, they're essentially the same.

So, my 100-400 comes tomorrow!

For me, the Sigma would not work.  I like to use my long lenses for not just distance, but also close up work, and the Sigma's minimum focus distance is 1.6m, vs 0.54m for the Panasonic.  Maximum magnification for the Panasonic is 0.5x vs 0.24x for the Sigma.  Given that there are no extension tubes available (yet) for L-mount lenses, that means I would be constrained to using diopters for the front end of the lens to get the closeup functionality that I prefer with the Sigma, vs being able to use the Panasonic  without any additional devices attached. So, for me, the Panasonic is the best option.  It definitely all depends on how you like to use your long lenses....(And the price of both is the same right now, at least where I am, so that's not really a part of the equation).

-J