Santa Monica, Costa Mesa, Newport Beach or San Diego? Looking to move...?

So I am from Toronto and am looking into going to SoCal for school... there are a few different options... I really have no idea what each of these cities are like. I am looking for a beautiful location, preferably close to the beach, with a pretty happening social scene and a lot of young people in the area. I realize it is pricey, but I am hoping to find roommates. It'd be great if anyone could tell me which city they would recommend and why and the differences between them. Thanks! :)