Heya,
I shoot a 300 F4 (basically) and a 150-600. I use both. I carry the 300 F4 + 7D on my hip. I setup the 150-600 these days more often on a gimbal and any camera attached to it, mostly for the long shots. I used to hand hold it exclusively, but more and more I leave it on a gimbal and use the much faster to focus and faster aperture 300 F4 for the close range stuff.
Both have their purposes. They don't really do the same thing. The 300 F4 is really good for close range fast moving stuff, so for me, that's birds. For small critters, the 150-600 is simply great, because 600mm makes a big difference when it comes to small wildlife, especially spooky stuff that you can't get close to. For stationary wildlife, I'd take the 150-600 because of the flexibility. Again, I use both though.
If you're shooting fast moving stuff, and you're at close proximity to the wildlife/birds, and you want action shots, and want to shoot in some of the lower light situations, the 300 F4 is a really great lens (the 300 F4 non-IS is a great lens for the cost, it's often only $500 basically). If you're setting up shop and in good light and want the flexibility to take on anything close and far, the 150-600 is great for that. Again, I just use both, because they do such different things that I want the two options at my immediate disposal when I'm out.
Very best,
:: My 
