A saildrive comes out directly under the engine, and is built to match exctly the engine inquestion. On the plus side it allows much more space where the propeller shaft goes on the other system. Gearbox, engine and drive all match exactly. It also removes a danger of leaking at the stern gland. But it is made of aluminium and there are often electrolosys problems and faults tend to need professonal repairing. Cost is also higher.