Tautline hitches or prusicks don't hold very well on the 1.25mm spectra, if you can even get that delicate knot done in the first place on that super thin cord, I was testing that recently. Works fine on his 1.5mm cord, and really well with 2mm cord like lawson (mountainfitter) sells, but 2mm weighs almost 2x as much as the 1.5 mm zpacks sells. These are all the spectra/dyneema cored, dacron sheathed variety both sell.
To me the difference between using knots and linelocs is whether you are pushing or pulling the cord to tighten it. It's easier to pull it to me. Which makes the lineloc a bit easier, the knot is fine but just not quite as easy to adjust.
If you weigh a lineloc and the required 1.25 mm cord, vs the 1.5 mm cord with knots, I think you'll find there's probably no significant difference in weight between the two.
I did manage to get prusicks knotted to 1.25 mm cord, but it was not consistent, and there's no way I could get it done with cold hands or other miserable conditions, and I wasn't able to do it consistently, nor did the knot hold consistently either, the cord is just too thin. Fine with linelocs though.
I think joe ships 1.25mm cord with his hexamids, not sure.