EF 300mm f/4L IS USM or EF 100-400mm f/4.5-5.6L IS USM?

[neuroanatomist]

Also with nature photography I assume you mean: long walks or hikes trough nature and shooting whatever ends up in front of you? Not sitting still waiting for subjects to enter your frame.

Yes...not planning on setting up a bird feeder in the garden with a tripod on the deck...

And I assume you've also got something like a 17-55 to do the wideangled shots?

Yes, I have the EF-S 17-55mm f/2.8 IS USM, and for times when that's not wide enough, also the EF-S 10-22mm f/3.5-4.5 USM.

A 100-400 would complete your collection I think.

Complete my collection (of lenses)? I had no idea that was even possible! [:P]