Help me choose Sigma 15-500 or Sigma 70-200 OS + 2x Ext

I would have to agree on avoiding the 2x....  I never used mine for a moon shot MAYBE it would have worked...

I shot with the Sigma 150-500 for a over a year.  I upgraded to the 50-500 wanting a little more versatility.

I don't know how Sigma made the 50x  THAT much better, but it is by far. Yes a few more coins, in my experience WELL worth it.