I personally would go Ultegra di2 only because I've been riding with it for about three years now and just can't see going back to mechanical. I've test ridden several bikes with mechanical Dura Ace and they shift very well but don't quite give me the same precise feel I get with di2. It would be easier for you to stay mechanical because you simply have to swap a few parts, adjust, and go. With a di2 conversion you're looking at swapping all of the parts, battery location, routing of e-tube wires, etc... Just looking on Jenson's web site, you can piece together a 6870 kit for right at $900 with an internal battery here from the US.