In reply to Rob Exile Ward:
I own a 4m cotton bell tent (heavy cotton maybe 340gsm) which is amazing and completely indestructible. It's been going 15 years so far with no leaks or reproofing and I fully expect to still be using it in the year 2050.
However, with three kids it's becoming slightly too cramped so I'm looking at upsizing to a 5m bell tent or equivalent sized Touareg (with bell tents you lose a lot of usable space in the eaves, the touaregs are better for this).
If space was not a problem I would always favour cotton canvas (durable, breathable, keeps you cool on hot days and warm on cold) but am now considering polycotton to help with packing the car. Hope some of this is of use!