7D or Sigma 150-500 Zoom

Here's my dilemma.

I recently sold my 7D & purchased a 5D2. I love the 5D2 but miss the extra "reach" I had with the 7D and my 70-300L. I find myself missing the ability to photograph those far away birds.

So my question is this. Add a 7D to my bag or purchase a Sigma 150-500 to regain my lost zoom range? I know the Sigma is the cheaper way to go, but what do you think will give me better IQ? The 70-300L f/4-5.6 on a 7D or a Sigma 150-500 f/5-6.3 DG OS HSM APO on a 5D2?

Thanks

For bird photography the 7D Sigma 150-500 is hard to beat, I've been using this combo for a couple of years.

I'm sure that would make a great combo, but what I need to decide is do I buy a 7D to use with my 70-300L or buy a Sigma to use with my 5D2?

If you have to make a choice I think I'd be inclined to go with a 7D, you also have the advantage of a backup camera then.