Depends somewhat on the strobes and controller you plan to use. Optical sync is simple, flood proof, and works well if you have a good match between camera and strobes. It requires your camera's built in flash to be able to open and fire inside the housing. You also need to be able to block that light from getting to your subject to avoid backscatter. On most cameras, again with properly matched strobe, you can have ttl strobe control with an optical link, by the strobe duplicating the light from the built in flash. Downside is, your built in flash fires every time. You use your battery a bit faster, and depending on camera, your recharge time between shots might be longer.
Sync cords transfer the electrical signal from the camera straight to the strobe (or through a ttl or manual converter to the strobe). You'll never have a dark shot because the strobe didn't react to the flash properly. The ttl signals are already built in, so the proper strobe/controller can read them and follow the camera's directions. You get consistent firing. I've never missed a shot with my hotshoe equipped rig....except....(Downside) when I didn't take good care of the sync cord. It flooded, destroying my ttl controller. New ttl controller, $300, new double sync cord, $180, time without the housing, 3 weeks. This was the first sync cord issue I've had, and it resulted from leaving the cord on the housing for 3+ months when I was hurt and couldn't dive. With proper maintenance, I'd probably still be using it trouble free.
