The basic grid takes each character along a very linear path, except Kimahri. He starts in the middle, so he can go almost anywhere from there. I don't know much about the expert grid, but apparently you're able to move all over the place. It allows for better customization, but you can screw up a lot easier if you're not careful.

Personally, I'd recommend starting with the basic grid, then starting a new game with the expert grid once you've gotten the feel of how all the abilities work, and think you know what you want each person to do.