My Timbuktu bag is heavy nylon and has seen a great deal of use and abuse, with little visible evidence. It has been in a heavy rain (for about a half hour) with nothing inside getting wet (the flap covers the openings well).

It is completely "civilian" looking, and even works well in a suit. The nylon is flexible enough to allow over stuffing a bit.

I did carry a canvas one for a while for certain activities, but it just did not hold up, and water soaked right through. Even after treatment to repel water, the stuff wore off in particular areas quickly and it even soaked up water from the ground.

Leather bags are heavier, but are durable and fire resistant, if that is a consideration.