Upload & Sell: Off
I previously had a 200 1.8 and 400 2.8 II (nonIS) and shot with a 1D3. Those two lenses make a nice combination with the TC1.4 and TC2 in giving you great span of reach from 200-800. The 200 1.8 does a great imitation of a 300 2.8 when paired with a TC1.4. I let the 400 go because I had stopped doing field sports very often.
Recently, I moved to a 5D3, and with the gained ISO performance of the 5D3 over the 1D3, and the lower reach, I made the move from 200 1.8 to 300 2.8 IS I. I am very happy with the move. My 70-200 2.8 IS II is nearly as sharp as the 200 1.8, and the 5D3 makes the results similar relative to ISO. WIth the 300 I get more reach native, and can get to 600 with a TC2, while the 200 would only get to 400 max.
A big factor to me was the lack of serviceability of the 200 1.8. I have very limited funds to spend on photography gear now, so if it died I would not be able to fund a replacement for a very long time. I didn't want to take that risk any longer. While the big whites are built like tanks and very reliable, accidents happen and things do wear out.
I'm very happy with the 5D3 + 300 2.8 IS I vs. the 1D3 + 200 1.8. That said, there are no bad choices among them... just slightly different pros/cons.