The Radians have a baller, true ambi lower; the upper, while nicely built, lacks the technical advancements that the SR-15 has, such as the E3 BCG system and the Mod 2 gas system. The specs of the upper also suggest that it's more oriented toward precision, given the quality stainless steel barrel compared to the SR-15's CHF'd chrome-lined barrel. The SR-15 has most ambi controls, but lacks the full ambi of the Radian (no right side bolt lock).
An SR-15 upper on a Radian lower would be money, IMO, particularly now that Radian has started selling stripped lowers again (I was lucky enough to snag the AX556s when they were still AXTS and didn't do full on rifles, still waiting on the AX762 though).