Speed density calculates the amount of air entering the engine based on temps, pressure, manifold vacuum, tps reading, rpm, and stored data covering the operating conditions the engine is most likely to see. Once the amount is calculated, it then calculates how much fuel should go with that, and then adjusts based on the O2 data.
Mass air systems actually try and measure the amount of air entering the engine with a hot wire anemometer. The companies moved to mass air systems because it made it easier to create cleaner burning systems to meet increasingly more stringent environmental/emissions specs.
The mass air systems will handle a broader range of mods than a speed density system will. And if you have to pass emissions testing, the mass air system is likely to allow it with a bit more ease.
There are plenty of good running SD cars out there. For mildly modded cars, there's no performance advantage of one over the other.