If it's about saving weight then it's got to be the 1.5mm - depending on your preferred length of guyline then you'll save 20 grams. Also the thinner diameter line means that you can use it to floss your teeth meaning that you can leave your toothbrush at home - even with the handle cut down and alternative bristles removed those thing really weigh you down (IMO). Interesting comments regarding the sawing action - I'll be looking into swapping some 1.5mm for my SAK (Titanium / drilled out and de-handled obviously)
At 1.66kg for the Solar I'd also look into taking some weight out of the tent - maybe lose a pole or two or remove the groundsheet?
The breakstrength of the 1.5mm is 110kg - it's not going to be the weakest link in the guying system
Ok, then the key to success is just buy one. Do you have a credit or debit card? If yes then WIN - google meet snowlion's fingers, snowlion's finger meet Google. The rest is just poetry in motion. If NO then I suspect that you may need to wait a while or ask your mum.