canon 300 f4 vs 100-400 f4.5-5.6
See this test of the 300/4IS that shows it is a nice, sharp lens
bare but loses a bit with a 1.4x TC (many, many other tests
on the web show the same thing....not that the 300IS with
TC is bad, many people like it, it just isn't as sharp as the 100-400 at 400).
The 100-400IS is a very versatile lens that is uniformly sharp from 100-400,
just not as sharp as a bare prime at any given focal length
The discontinued 300/4 non-IS is sharper even with a 1.4x TC and can
be had for between $600 and $900 used
No IS so keep your shutter speed up at 1/f or more or get a monopod/tripod.
The 400/5.6 is cheaper than the 300IS and the 100-400IS and is sharper and
has better contrast than either at
400mm. Gotta keep your shutter speed up, etc...
though. Great for wildlife, especially birds.