My 300tdi has coaming on the sides but not rear.

The Td5 is flat - buts got the canopy permanently mounted to I've not really had the opportunity to 'experience' it ie. I can't really compare.

The downside for me is that the canopy for the 300tdi is as wide as the tray and so has to sit on top of the raised edges, rather than within, and there is far less surface tension for the box and nothing holding it in. As I use over centre fasteners to hold it down (I know, I know) ... this matters. I have reasonable tension vertically, but its wildly insufficient horizontally - not helped by the large load being spread on a relatively small surface. Not good for wide loads.

When the canopy is off, I find having the stop for things being tied down very helpful. Its never really been an issue, but could always slide something off the back if it becomes tricky to lift over the edge.