Who Chose the 300 f4 over the 100-400 for Birding and Why?

I splurged and bought both. (Though I bought a used 100-400 and have not used it yet). I have an E-M1 and bought the 300 mm first because it was the one that was first available, and I was going on a trip. My current plan is to use the zoom when I am in a vehicle (car, bus, or boat) or expect to be photographing lots of small birds in flight, and the 300 mm plus teleconverter when I am on foot (when I can zoom in or out with my feet).

The advantages of the 300 mm plus teleconverter are slightly better optics, slightly faster at 400 mm (significantly faster at 300 mm), slightly more reach, and dual IS with my E-M1.  Easier to switch between manual and autofocus while focusing (you just slide a ring with the Olympus but have to fiddle with a switch on the Leica); 1.4m to 4m focus limiting option is good for close birds, insects, etc.  (Leica only has a 5m to infinity focus limiting option).

The advantages of the 100-400 mm are the ability to zoom out and shoot larger mammals (rather than just their heads) that are close to a vehicle, seabirds flying about a boat, quickly get birds in flight in FOV before zooming in, or cetaceans close to a boat. Also lighter weight (though I've not really had any muscle soreness problems with the Olympus if I carry it on my shoulder).

If I had to choose just one, it would be close but I'd probably go with the 100-400. I might be tempted to switch to a panasonic body to get dual IS though.