I would choose the 7D for a few reasons:

1. The effective maximum aperture of the 5D Mark II combo will be f/5.6 X 1.4 = 7.84, nearly f/8. This will somewhat cancel out the light-gathering advantage of the full frame camera.

2. You will still have a bit less effective reach with the full-frame camera, even considering the small pixel-count difference and even if you crop. Cropping always degrades image quality as well.

3. While the 400 f/5.6L is not a bad prime by any means, its image quality will noticeably suffer when using the 1.4X teleconverter (mouse over the image to see the differences), further reducing any seeming advantage of the full-frame setup. You would not much like the results with a 2X TC, let alone stacking TCs.

4. If 560mm is not enough reach, you will have "nowhere to go" with the full-frame setup, whereas on the 7D you can still slap on the teleconverter and likely get a much better result than you would by cropping the full-frame image. (And, remember, the 7D without the TC will have a bit longer effective reach than the 5D Mark II with the TC.)

5. Optical differences are hard to assess without comparing both in the field and filling the frame with the intended subject. Note however that besides the optical impact of the TC noted above, the 7D + naked 400 f/5.6L would be expected to have more consistent edge and corner performance than the 5D Mark II + 400 f/5.6L + 1.4X TC. Balancing this "sweet spot" effect of using a crop camera is that the lens will be challenged a bit more by the higher pixel density of the APS-C camera. Still, with such a high-quality prime, you will still get quite excellent results on the 7D wide open.

6. The 7D offers other advantages in autofocus, frame rate and build.

You may also note that the pixel-count difference here is nearly negligible, only about 300-400 pixels in each dimension. For all these reasons I'd lean toward the 7D, at least without a chance to field-test them both.

使用擴展器通常會導致圖像質量下降，而其他所有條件都是相同的。

其他條件都不相同的情況是弱光。您不能自動假設全畫幅會更好。這將取決於傳感器。

如果您有足夠的光線並且受到玻璃的限制，根據我的經驗，作物傳感器將提供更好的圖像。

鏡頭加轉換器為f8-不會專注於5dII。我認為只有f8 AF的機身是1dx。

即使它確實對焦，停止慢一點也能吃掉FF的優勢。

轉換器在光學上從來都不是完美的。它們會柔化您的形象。但這通常不足以解決問題。

轉換器對作物傳感器有意義的唯一方法是，如果它使最終圖像中的像素明顯增加。 5dII僅比7d具有3mp的優勢，即使這樣，它在很大程度上也失去了1.4倍與1.6倍的差異。

我選擇5dII加轉換器的原因之一就是歷史關於7d的AF問題。但是，如果您的AF表現良好，則在這種特殊情況下使用FF沒有任何優勢。

I'm not a bird photographer and I never had such a long lens, but as I understand the Full Frame 5D mkII should give you much better quality in terms of light, considering that you're using a f/5.6 lens. but on the other hand the 7D is a much faster camera, with 8fps, and it should be very useful for fast moving subjects.

5Dmk2 is a 21MP camera, that's 3MP more than 7D, so you can always crop the images and it should cover your need of a longer lens.