UI guess it depensds what your main mooring place is. The "D" section ones are good for flat sides (e.g if you are alongsiode a ship a lot) as they hold the rest of the toobs well clear. "Flat" is better for marinas, because you will find the "D" section ones on an SR will be just below the pontoon deck as you come alongside, and will catch as you strep out and the hull floats a bit higher.
As for the triple, Mine has (had, as half of it has fallen off) a "D" with a"flat either side along the whiole length. Pic is the first one on the "show us your humber" thread in the "pics" section, but note lengths of it have fallen off (the boat is 27 YO!) .
Walking panels - Mine just had a big patch of Hypalon glued to the top of the toobs. Also had 4 strips of "ribbed, flat" rubbing strake neatr the A- frame as a step point, which donlt show up too well on the pic.
There are many ways of doing this!