The mercury versions emulate specific chips that came in the cartridges to add certain functions to games, (think starfox, mario Kart) the standard versions you need to provide those chip dumps yourself. As for the performance, balanced, accuracy they are just that, if you have a powerful system then accurate would be the best version, but it needs a high clocked cpu, balanced is nearly as accurate but uses less cpu, and performance is again less accurate than the other two but uses less cpu than the others.