See I think Cumberbatch would have been way too similar to Smith. I actually think someone like Capaldi would have made a good Master for Smith, as Moffat says Smith is an old man in a young man's body whilst Capaldi will be a young man in an old man's body, and that's the dynamic Id have like to have seen....
The only similarities between Cumberbatch and Smith would have been their age and height. Cumberbatch's Holmes and Smith's Doctor are similar, but both actors have range beyond that, and Cumberbatch wouldn't have been playing the Master as Sherlock, certainly. The physical similarity would have added to the mystique of their differing personalities. At least, that's how it looks in my mind. Something more like his John Harrison persona, but with much more charisma and personality.