Apple's own visualizer is of course a modification of G-Force. I like Apple's album art display, simpler controls, and speed, but the "real" G-Force has more features otherwise. Resolution control, more effects, etc.
The "real" G-Force has been out for iTunes forever--but do get it, it's fun! If I could only have one visualizer, this would be it.
Luckily I have 14 visualizers currently installed 🙂