If you look at the statistics, come playoff time, 95% of the production comes from a team's best 6 players. The rest of the roster has virtually no long-term impact over the course of a series or multiple series. In that sense, it might seem like being top-heavy is better. However, teams in the NBA, by and large, are very bad at figuring out who to give minutes to outside of superstars (look at the Denver Nuggets, who give minutes to a woeful Wilson Chandler and almost no minutes to their best player, Kenneth Faried).
So my answer is ... well, both. You need a core of great players, and a lesser core of decent players, to win a championship. Every single championship winner has fit this mold, actually if you take the time to actually look at the statistics; it is just that decent players that fill their role are not as memorable as superstars, so when we look back retroactively it is hard to remember the 4-5 best players on the team who were instrumental in the ring.