I assume you have the older S2, not the version that's out now. Drilling holes in it is going to lose value on the frame if you decide to sell in the future. Therefore for me, I would go with the 9000 group.

You can also use the new control box on the 6870 as well. But, VPR is correct, that is the original control box for the 10 speed.

I would recommend 9000. I love Di2 and have had 6770 and now 9070. I think Di2 is amazing but I don't like the look of Di2 run external. Go mechanical on a mechanical frame.

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Ridley Noah FAST = 6.86 kg
Ridley X-Night Disc 7.80 kg
Raligh HiLife = 7.46 kg
http://weightweenies.starbike.com/forum/viewtopic.php?f=10&t=104672

I do have the older S2 well 2013, which means I would only have to drill 1 hole in the top tube and then it would be internally routed. I wouldn't run Di2 externally as I agree it doesn't look good at all. I wouldn't normally consider drilling a frame but it cleans up the S2 cable routing a lot.

I'm currently swaying towards dura ace mechanical and will save the Di2 for another bike maybe.

Went ahead and got both. Dura ace is going on the S2 and 6870 is going on my Rose Xeon RS.

Could have just went with DA 9070 for the S2 but this way both bikes get an upgrade and I'm fully 11 speed with my bikes and can swap wheels