One is with the Canon 7D and 300mm f2.8 ll and 1.4 T.C.
The other is with a Pentax K-5 and Sigma 500mm F4.5 Prime.
After close inspection it is obvious the 300 mm Canon 2.8 ll produces a beautiful pic, but it is not near as sharp as the Sigma 500mm lens.
So for birding the 300mm F2.8 lens performes exactly like the review at DXO marks predicted. It is a good lens but a poor resolving lens. And for birders that means death.
So in my opinion not a good lens at the asking price ( For birders ). I only shoot birds so I cannot comment on other uses.
Much better off with a MUCH cheaper Sigma 500mm lens any day for birding. This is for detail and resolving power. The 300mm offers a nice improvement in hand holding (IS). And will bring home more keepers hand held. But not as sharp with a T.C. as my particular Sigma 500mm F4.5 prime.