There's not just one model of either the SR-15 or SR-16 to simply answer that question. And historically the SR-16 has retained certain characteristics not found on the commercial offerings such as different barrel profiles (typically medium), different barrel contours, ball mill dimpling, rail systems that have not been commercial offerings, full auto carriers even when the commercial SR-15's to include the earliest production E3's had semi-profile, enhancements such as chromed bolts that were made with various exoctic materials (they have not all been the same), other features may include the chromed sand-cut carriers which are a latter feature, they have always also maintained standard MILSPEC MIL-STD-1913 railed Upper Receivers vs the commercial offerings that have on rare occasion had the integrated rear sights . However keep in mind that the features are typically customer driven requirements (those customers being US and foreign government entities), so one entity may specify chromed bolts and another may not for example. Some of these features will sometimes migrate to the commercial offerings of which we are lucky to benefit from given the R&D and T&E costs to bring those enhancements to market likely wouldn't have existed if there wasn't an existing USG requirement or funding source.
ETA: If you drew two parallel lines with the top being the SR-16 and bottom being the SR-15 at no point during the last 10 years have those 2 lines come closer to converging than they have now.