I'm riding 2013s for both models. I was told there was a difference between the years, but I felt nothing other than my 13 Bill is more broke-in.
The William is a fierce machine. It is amazing how much more power you can get off the wake just by changing the core materials. That said, It is a little stiff for the park. I have buddies that roll with it, but I definitely prefer the Bill at the park. Now, if I was to only choose one for both, I would probably go Bill.
Below is all my opinion:
In the Ronix line, the two easiest boards to ride are the Bill/William and the Code 22. They are both very constant, predictable and pop well. Where they diverge is that the Code is built more for riders with smooth progressive edges on all tricks. Wake to wake or to the flats it likes a smooth turn and a driving edge through the wake. The Bill/william on the other hand is a little more dynamic. It can accelerate and brake faster than the code and goes edge to edge a little quicker. I've ridden both back to back and rode both great. The Bill just happens to suit my style a little better. My buddy (expert rider) said the exact thing about the Code. I've ridden all of the other boards in the line up (less the new Parks) and they were all fun to ride, but the other shapes were a little more specialized and built more for a specific type of rider. IMO, the Bill/Willam and the Code cover more riders well.