I actually have that rug its a great canvas it did stretch slightly at the neck but I'm really happy with the quality.
Canvas and synthetic rugs do different jobs. I like to think of it as a jumper and a snow coat. Ours get a canvas on when it starts to get chilly April usually I think.
Then when it gets COLD they move to synthetics. If its NASTY they might even get both!
Synthetics often don't breathe so they can sweat in them if you're over rugging. Canvas breathes so they don't tend to sweat as mch.
I have this rug on prin atm AXIOM 1800D BALLISTIC GRN/BLK 300g CB - 5' 9 | eBay
Im in melbourne btw
I wouldn't look at a synthetic less then 1200denier and love these ones. Have one in its 4th winter in a barbed wire paddock on a gelding who plays constantly and plays ROUGH. Its just got the smallest pucker.
Ideally you want both a rug for when its chilly and one for when its COLD
Neck rugs on synthetics are good but a personal choice. If its a bit warm removing a neck rug or even unbuckling it will help them regulate body temp.
All of this depends on your horse, its breed, age etc
I hope this helps feel free to fire more questions at me.