Sigma: trading 70-200 f/2.8 for 100-300 f/4

I currently own the Sigma 70-200 f/2.8 (newest non OS version). Since getting my 400L, I have been using this lens on my second body for a 70-200/400 widlife combo. I personally prefer this approach for the faster autofocus and more dof control across the range.

My question is if I should trade the 70-200 for the 100-300? I have found that in situations where I need the f/2.8, I also need IS and end up renting the 70-200 f/2.8 IS anyway. From what I've read this lens is just as good as the 70-200 and would give me a bit better range coverage for wildlife. How do they compare in prices? Could I sell my lens and pick up a used 100-300 for a similar price?

I just picked up a like new 100-300 DG from Adorama for $900 but was looking at a non DG on FM for $700 not sure what a Sigma 70-200 fetches but I think it's around $700. If you have patience you can find a 100-300 around $600 to $650