I'm going with the License Board, it just offers greater flexability than the Sphere Grid. As others said, with the Sphere Grid your fairly confined to a specific path for most of the game. It isn't until around the point you get the airship that you get the apropriate spheres to develop your characters the way you want to. Even at that point you still can't pick what weapons and equipment you can use, though since most of the Final Fantasies since the PS era started don't allow you to pick their weapons that isn't a big deal for me.

License Board on the other hand gives me complete free reign to do what ever the h*ll I want with the characters. I can make any characters any job class, I can deviate from the jobs classes and make characters with just flat out crazy set ups, or I can make my characters walking demolition crews and complete the board, its all up to me. I can pick and weapon, armor, and accessory with any character. This is easily one of my three favorite development systems (with materia and the job class system from Tactics up there with it).

Don't get me wrong, I like the Sphere Grid, X is easily one of the only RPGs I actually like to grind in and its all because for some sick and weird reason I like having my characters traverse the Sphere Grid. However, I just love the flexibility of the License Board a lot more, when I play RPGs I like to have quite a bit of flexibility with how I develop my characters, and XII's offers more than X's in that regard.