Mel Gibson has directed two big hits and one minor one. Many directors can say the same.
Jackson has directed a trilogy that's been beloved for decades and has a huge fan base. Don't get me wrong, I loved the LOTR movies and never read the books, so I'm not slagging the guy. But there are enormous flaws in the trilogy, which is probably my favorite fantasy movie (I count it as one, as does Jackson).
Potential major competition? Sure. But come back in 30 years and we'll see. Spielberg's been directing huge hits since the mid-seventies, and Jackson and Gibson probably won't even live long enough to topple his record.
On a personal preference, I prefer those two to Spielberg, but they're not even in his league.
My point was that Jackson and Gibson each have few, if any, constraints on what movies they make and how they go about making them.
Gibson because he has the enormous bankroll he got from The Passion, which means he could make the biggest-budget movie ever out of his own pocket if he feels like it. Jackson because he has made three in a row of the biggest hits in recent years, although KK may or may not live up to his previous performence. As such, either of them can have few constraints on what movies they make.
My comments were not as to whether either of them is an artist in Spielberg's league.