As rvinc said, si only shows what is to either side of the boat, and directly under. 360 does just what the name implies, it shows a full 360 degrees, EXCEPT for what is directly under the boat. You can't use the 360 transducer for SI or di, and frankly, you wouldn't like the results if you could. While the 360 allows you to see structure and large schools of fish within 200ft(I think) of the boat in any direction, the images aren't nearly as clear as on di or si, especially when trying to identify structure, or seeing smaller groups of fish, or seeing fish in brush or other structure. So really, I wouldn't suggest buying the 360 to replace di/si, but rather to use it in conjunction with the tools that you already have.
You can still use di/si with the unit, you just have to use a separate transducer. I hope that made sense?