Between the two? Probably M855. As noted you never know for sure what your going to get with loads that are only marked SS-109. M855 shouldn't cause you any problems and is generally well liked by AR15 shooters. Don't worry if ammo is labeled as SS-109/M855. That should be M855 spec.
Note: M855 is effectively a implementation of the SS-109 interoperability standard (so all NATO members can shoot each other's ammo). The US, however, requires stricter standards in M855 and as a result, M855 manufacturers generally load their rounds to hit at least 3000 fps at 78 feet from the muzzle. The SS-109 specification had a lower 2985 fps requirement and British SS-109 rounds are slower still (2700-2800) to deal with the SA80 rifles.
Opinion: Some British SS-109 reportedly is underloaded (in order to permit proper operation in the L-85 Bullpup also called the SA-80 rifle) and therefore causes some short cycling in Bushmasters and Colts and isn't likely a good choice for emergency or critical use ammo.
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