They both have different uses. Lv Max is mainly handy when setting up benched LVX's that you dont plan to attack with. Premier Ball is for those who need their LVX to attack.
How is this a Rules question btw?
Also, Premier Ball is mighty handy when you need to get an Lv. X back into play from the Discard Pile. More so than Night Maintenance + Level Max, at least.
Same as Lou said, Level max is for setting up bench Lv.X's like Heatran Lv.X and Uxie Lv.X in my case, and primier ball for for Lv.X's Like Flygon and other Lv.X's that are meant for acually fighting.
I used to be all over level maxes because they make it ultra easy to level up a bench sitter. I've really shied away from them recently though because it makes your deck less consistent. You can have a handful of level maxes and get nothing. Where as if you have two switches in your hand, you KNOW you'll be able to get the X on your bench.