Weight is the big advantage of the ripstop models.
In speaking to the Bison guy, the canvas is stronger, spark tolerant and breathable but is twice (ish) the weight. For me, it's the 'feel' of canvas too.
The hydrostatic head (vertical column of water) of the material is far better for ripstop but the shape of the tents will mean there's no puddling anyway. If your tent is under 4m of water I would suggest a different camping spot....
Unless you need to carry your tent some distance, the general advice is go for canvas.
Other comments welcome, PLEASE! I started a thread on this very subject over on SotP prior to deciding (foolishly) to set up a group buy - Me?, I'm going to get a canvas Lavvu!
Let me know and I'll add you to the 'interested' list - no commitments at this stage.
Cheers
Jon