7800 shifter is for the the rear
7801 shifter is for a double.
7803 shifter is for a triple.
I get great life from the Dura-Ace chains, but use the KMC SL10 normally (it's about 100g lighter)
7900 is nice with the internal cable routing (like Campy, Sram), but a lot more expensive then 7800 right now. Sort of depends on what you want.