Neither of those. There's some sort of committee that selects the group to participate. Generally they choose people with statistics to back up the selection combined with some measure of popularity in some cases to add interest to the event. Sometimes there are exceptions...for example, after Craig Hodges won it twice, he was invited back even though he was actually out of the league (I believe?) when it came time to defend his title a third time. I can see them inviting someone with a reputation for being a gunner all his career...even if that guy was older and somehow stuck near the end of a bench this season. But a rookie with no rep, no playing time, and no history in the contest? Ain't gonna happen..
The guy who won the year before automatically gets an invite, so Dirk's in. Guy with the highest % is in (Luther's 4th, I think). Guy with most makes is in (Luther's 3rd). And then it's a random bunch of guys that shoot a good percentage. Luther should make the contest, but he likely won't win because of reasons mentioned by earlier posters. Still, never count out a confident shooter.