With the fixed length 300 you'll be limited in compositions and I don't know how easy it will be to "zoom with your feet." My 100-400 works great, and the IS allows me to hand hold a lot of stuff. If I need shutter speed I bump the ISO. You get excellent high ISO performance with your 40D.
I've never been to the Galapagos, but I've done lots of international travel and I'd pick the flexibility of the 100-400 over the large max aperture of the 300.
I'd also beg, borrow or steal something wide. The 16-35 or 17-40 would be great. Your 24-70 will be excellent in Quito, but something wider would be good, too, as well as the landscape possibilities.
Most importantly, don't worry too much. Whatever you take, you'll have great photo opportunities.
You don't mention if you have extra batteries. I would take plenty!