Admittedly, the two studies aren't
perfectly comparable. But, I think one can see the difference in heat reduction trends between a solar reflective roof and a radiant barrier.
If the ultimate goal is to reduce solar heat gain in an attic I think that a solar reflective roof surface would likely perform better than a "standard" roof surface with an underlying radiant heat barrier. The logic being that it's better to reflect solar heat from outside the roof structure (reflective cool roof) rather than from the inside (radiant barrier). That said, I'm certainly open to data to the contrary.
Of course, if you have a new(ish) roof surface a radiant barrier would probably be more appealing. But, if you need a new roof then choosing one with a high Solar Reflectance Index shouldn't cost any extra.
Here is a link to a manufacturer's metal roofing color charts with SRI ratings -
Metal Sales - Colors (I have no affiliation with the manufacturer). I think it's interesting to compare the range of roof colors to their SRI. The color charts show that a cool roof doesn't need to be white.
FWIW,
Tim