The best answer is… It depends. I/Os are the often the least expensive, and you have the advantage of having a realtively car like (simple) installation. You can get to the engine pretty easily in most boats. The disadvantages are that the outdrive can’t tilt clear of the water, so if you boat in saltwater the aluminum outdrive is always submerged and subject to corrosion. Also, the rubber bellows that encloses the drive shaft, as it comes out of the boat and into the outdrive is usually at least partly submerged. A hole, or tear, in it can sink the boat.