Allow me to explain
Realtime is done in real time, on the spot, by the console. In many instances you can possibly move the camera during realtime sequences or even controll them, however the level of detail cannot always be achived. They also take up ALOT less room on the DVD/CD
CG is a computer generated movie aka prerendered. Its done before it reaches the console, and then the movie is played, just like a DVD or VHS. The data is already caculated and ready. Problem with this is beacuse its preset it cannot be changed and is always the same, but you can reach higher levels of infinite detail. But this also takes more room on a CD/DVD
There is really no best, some are used better in some situations.