You should post this kind of thread in General Discussions since most of the Users here are in that section as you are asking for advices and not reporting bugs or giving suggestion for the game ..

But okay, for your topic, better go for Lucky Board than spending in Wheel of Fortune .. If you only targets the ninja in the Wheel then it is not recommended to spend your coupons there because you may end up with nothing since that Wheel event involves RNG or in plain word, Luck .. While on the Lucky Board, everytime you throw a Dice, you will get a certain item for power and a point that you c@n use to redeem something from its redeem shop .. A power-up item or ninja fragments ..