From everything I've ready, the 8000 series Di2 is fully compatible with 6800. As others have pointed out, the biggest difference is going to be the rear mech. I was able to put together a 6780 upgrade kit (I already had brakes/crank/etc)with the medium cage RD for about $700 last year ordering from the UK. I swapped the cage with a short cage from a mechanical 6800 and have been extremely happy with it.
Honestly, if you have the money and like the newest/latest/greatest, go with the R8000, but if you can throw together a 6870 and save a lot of money knowing you can add R8000 as wanted/needed, personally I'd go that route.