Which is more accurate: mercurial or aneroid sphygs?

By law, both instruments must initially be within 3mmHg of a reference standard. However, because aneroid gauges rely on gears and mechanical linkages, mercury instruments will likely REMAIN accurate for a longer period of time. However, there is a trade-off; mercury instruments are generally not as portable and necessitate the use of 99.9999% pure elemental mercury, which is being banned by more and more municipalities and states.