I ummed and aaghed about this when I changed my canvas.
martyboy on here has the JK one, which he is (I believe) happy with. I looked at this, and it is like a modern tent fabric - so pretty thin but probably very waterproof. It looks very neat.
I went for the VW Heritage one, which is more traditionally canvassy cloth. I'm very happy with the weight of it and the look, but have yet to get rained on. I don't think it will be as waterproof, as I can see very small holes through the stitching - I suspect it may be a little too taught, but I wanted to avoid that saggy look. It was a tad big at the base, so I have a little "gub" at the front corners.
Incidentally, all of the instructions I have seen say you must remove the roof to change it. Well I didn't - I couldn't see why you had to when you just tack it on on the ground, then fix it properly in situ. And unless I have put it on totally wrongly, it was fine to do with the roof still attached.
Cheers,
Nick