in terms of aerodynamics, durability and availability/cost of replacement parts, what would be the best choice considering I weight 76-80Kg and I can get a second-hand pair of each for about the same price?
I've got a couple pairs of 21's. I weigh 160 pounds and they have been great training wheels over our rough roads.
Aero? Meh. Anything under 30-35 MM isn't on my radar for aero. I haven't had to replace anything on them so no clue how shimaNO's pieces parts stocking will be when/if it comes time. I would recommend them as good daily drivers.