For the amount of kit you are planning to put in it, I'd definitely recommend the 445 over the 335.
My own method of carrying kit in Billinghams did not involve leaving a lens mounted on a body, so I can't comment on how easy that will be to do. If you plan to put the camera/lens combo nose-down into the bag, then the 445 is definitely the best, because it is higher; however, I'm not convinced even the 445 is tall enough for a body + 70-200 combo nose-down. I'd suggest the 550 for that, though you are starting to go big and expensive then!
If you choose to keep the body and lens together, placing the combo into the bag lengthways would work, though it takes up quite a lot of space inside a 445, leaving little room for the extra kit you wish to carry.
Personal background: I owned until recently 3 445 bags, a 335, and a 225. I sold off the 445s and the 335, making the switch to Lowepro backpacks, to allow me to carry more, and to reduce back pain (the load is better distributed with a backpack, I find). I loved the Billinghams, and once on site, they were great for getting equipment in and out, and moving around without having to fully pack up each time. It was getting the bags to the location which was hurting me. A fully loaded 445, with strap slung across the opposite shoulder, plus a big tripod, was making photography a drag. The switch to Lowepro has worked well for my needs, though they are nowhere near as watertight.
Given a choice between Nytex and Canvas, I'd go for the canvas every time. Also choose Khaki colour, for reasons of resale value and light reflection (dark bags absorb more light, and thus the contents get hotter).