For the vast majority of your shooting you can't beat the 500mm with a 2x entender. You can encounter a slowdown because you are giving up two stops with the extender. Also your best results come with this set up when shooting at a reasonable distance like 100 yard, the detail are amazing.
For fast moving subjects like a bird the 2x will slow your auto focus. so pre tracking and following the subject is really important to have a sharp image.
One last point, you will lose auto focus if you aren't using a pro body with the f stop above 5.6.
What exactly do you mean by a pro body? I have a 7 D model