I don't use either, my fuel line on the 250 runs right down next to the thermostat housing and valve cover and works just fine, it has never vapor lock yet. I would bet the shinny flexible sleeves would do a much better job then rubber, but the rubber would be a cleaner look. The flexible sleeve would need a air gap inside to work properly.
At work, school bus garage, all our Cummins 8.9L and 6.7L engines run on compressed Natural Gas. They run extremely hot and use the silver shinny sleeve material on a lot of the wires and cables. If Cummins thinks its good, it probably is.
