I've used all three brands so here we go.
I always used Rubi's with no complaints until porcelain tiles arrived. Sol i bought a TS60+, the bars flexed so it didn't cut straight, also the breaking mechanism was rubbish for porcelain.
So i bought a Montolit 750. Took a while to get used to not seeing the marks as easily as the Rubi, but cut everything i threw it's way. The great thing is you can set the gauge and move it up out of the way for other cuts, return it when you need the gauge, really really handy.
But it snapped around the pivot that alows odd angled cuts, so i was doing huge commercial floors at the time, and bought a Sigma.
It cuts better and thinner than the others without a doubt, so that's good. But its cutting mechanism is a little fiddly and isn't as smooth on its rail as the montolit, also the gauge isn't as good as the Montolit as it is fixed. It's heavier and seems built to last, but i do miss the Montolit.
Now i've not tried the single rail Rubis so i can't comment, but my mate bought one and loves it.
So summing up, i thought all in all the Montolit was easier to use all round, and was lighter to carry. The Sigma is solid and cuts everything, but isn't as smooth, and the TS was sold and i don't miss it, but i use i TS40 all the time for site work.
Many tilers refuse to try any other brands and are totally stuck in their ways which is just stupid, buy the best tool for the job, no point being a hero battlling with a crap machine just because you like make, you could have finished hours earlier and be sat at home having a cup of tea!