6mm might be a tad lighter but it requires more work from the pump thus more power from the battery.
There's 2mm in it! you'll still get 100m of 8mm on a standard hose reel.
8mm for me coupled with a digital varistream. Only requires the pump to run at No-1 flow setting & will run direct from van battery all day! 
There may only be 2mm in it but the important measure is area, not width.
6mm hose cross section is 36mm sq, 8mm hose is 64mm sq.
100m of 6mm hose will hold 3.6 litres of water. For 8mm hose, it's 6.4 litres, so it'll weigh nearly 3kg more in addition to the extra weight of the hose.
One important point, though, even on the biggest places I do, armed with a van mount system, so moderately inflexible, I NEVER use 100m of hose. I very rarely go over 50m. I'm thinking of cutting down to 60m and having 40m extension on a cheap hoselock reel for the one clean in a week that needs it. My work is pretty much all domestic, which might explain it.
Vin