Help with wild cards

Hey guys, i just got 50k wild cards that means im gonna get 50k xp and will need only like 15k xp to be lvl 54. I wanted to max my firecracker to lvl 15 but im wondering if its not better to max my princess tower to lvl 15 so i can use my booster and play with my whole deck on lvl 15. but im not sure, is 50k wild cards guaranteed from the wild chest you get when you reach lvl 54? I would still like to upgrade my firecracker to lvl 15 once i reach that lvl 54. please help me