Last year when I was buying a new rope system I did the math on this whole single, twin, double debate. Single + tag line is the absolute lightest IF you buy the spectra 5.5 as your tag line. Keep in mind this will cost you about 2x the cost of a single line. Twin ropes will be lighter if you opt for a single + 6mm tag line. Double ropes are definately heavier.
Here's the math:
Mammut Serenity 70m @ 52g/m = 8 lbs
Spectra 5.5mm 70m @ 16g/m = 2.5 lbs
6mm cord 70m @ 26g/m = 4 lbs
Mammut Phoenix (dbl) 140m @ 41g/m = 12.7
Beal Ice Twin 140m @ 37g/m = 11.4 lbs
Single + Spectra = 10.5 lbs
Twins = 11.4 lbs
Single + 6mm = 12 lbs
Double = 12.7 lbs
Some newer ropes may have come out, but when I did these calculations these were the lightest ropes for each catagory I could find.
Keep in mind there is more to a rope system than just the weight. Different systems are better for different routes.