If the California Grill menu is more appealing to you, then you should stick with that one. Flying Fish, however, is amazing. It's one of our favorites... we actually went twice during our last trip.
Something to keep in mind, if you are driving, is that you are not likely to be able to park at Boardwalk unless you have a dining reservation. So, if you were planning to drive there to check out the Boardwalk then you might want to have the reservation so you can access their parking lot. If that's not an issue then I would go with the menu that looks best.