Ok here is the skinny of the different processors. A speed density processor runs on a fixed set of values for the stock engine to run. The program is pretty much fixed and takes its readings from the MAP, TPS, ACT and the HEGO sensors. The program works for a specific amount of air charge and fuel metering and if you excede this threshold the processor cann't compensate. Lets say you want to add a cam or a nice intake increasing the air flow into the engine. You end up with a rough idle or constant stalling and it will burn nice and rich robbing you of any power increases you attempt. In a sense the processor crashes.
Mass air works with one sensor domanating the input. The Mass Air Flow Sensor. This sensor measures the amount of air flow and the temperature of the air. This system will adjust to increases of air coming into the engine, so if you add a cam the processor will compensate for it rather than crash.
Now this info doesn't neccesarily mean Speed density is bad. On a bone stock engine I think speed density is better. I used speed density on a buddy '82 Bronco with a Mustang 5.0L engine and I think it was the best choice for that application. If you plan on staying stock go with speed density but if you even think you may modify the engine then Mass Air is the only choice.
