Like someone mentioned, I wouldn't do double blue mage; use one or the other.

The advantage with Robin(M) is that you can bait units with him, which makes positioning easier. Reinhardt's speed is pretty much non-existent, so he gets doubled by anything that has a chance to attack him unless you put a -breaker skill on him, and often he'll die to it.

This means Reinhardt needs to initiate without getting killed by other units afterwards, which generally requires either a dancer or a teammate with Reposition/Draw Back.

The advantage with Reinhardt, aside from his mobility, is that he has the potential to one-shot everything that's not green.

5* neutral attack Reinhardt with Death Blow 3 can one-shot pretty much any Blue or Colorless unit you see in Arena except Nowi, Effie and Olwen on initiation, and of course he stomps Reds. If you're lucky enough to have a +ATK Reinhardt with DB3 he stomps ALL neutral Blues/Colorless/Reds, as far as I can tell. If you have a +ATK Reinhardt with DB3 and run an attack buff on your team, I think he stomps all Blues/Colorless/Reds regardless of IV, which is pretty awesome coverage.