e4 or d4?

Theory is there in both e4 and d4. However if you want to avoid theory, a good thing to do is to learn systems, for eg london system, kings indian system etc where u arrange your pieces in same way and have same plans usually irrespective of what your opponent does.

Play both but pick one as your main weapon.

I liked what Jan Brown said a lot.

But is 1.e4 really te safest approach nowadays? On the candidates this year, many guys like Karjakin, Svidler and Caruana (who plays e4) were trying somethink with the English opening (maybe something "original").

And what about the match Kasparov vs. Kramnik? The first played 1. e4 and couldn't win a single game, while Kramnik playing 1.d4 won 2.

What I'm trying to say is that I know that e4 has tons of theory and always very sharp. But it seems that with d4 the game gets a little easier due to the "lack of possibilities" because of the closed game.

So, what's the point in getting to know tons og theory in e4 when you have d4?

By the way, thank you guys for participating on this nice debate.

But, at the same time, it's a little funny to play d4, because it's an opening that closes the game and makes it a little easier.

If it gets easier and also with lots of play for white, why do GM like Caruana or Karjakin prefer e4?

KingCrusher has an excellent video that says something about this "Why play main lines?"

Why would somebody learn everything about Ruy Lopez nowadays to meet the Marshall gambit?

Is it because they only "know how to play" 1. e4 and feel safer playing it?

I play Pawn to King 4.
It provides me with a long life.

@SouzaMoraes why do lines become main lines? Why is the Marshall Gambit a main line, but the Blackmar Deimar Gambit considered at most a footnote? The reason is that main lines are lines that generally give the best chances for both sides. For white to fight for an edge and for black to retain dynamic equality. Whims of popularity and trendiness plays some role, but in the end mainlines are going to be the most meritorious openings.

As for e4 or d4, I'd recommend e4. I think it's the better choice for improving. Closed and semi-closed games require an appreciation of the open games to be most effectively played. e4 tends to lead to open games, d4 tends to lead to closed and semi-closed games. e4 also tends to more regularly lead to a wider number of instructive and unique pawn structures.

@SouzaMoraes
It is not that easy. At the start of the tournament with superGM's nobody knows how each one of them is going to perform. So they are being cautious and when they smell blood or if they realize that someone of them is momentarily out of form they are going to go for him. But they dont want to heal him by losing their own game with a hyperagressive play. They need to find balance and that is their task. Because each one of them is capable of winning a big tournament.
Flexibility of openigs is important too. If you play only 1.e4 or 1.d4 you become predictable. Both 1. e4 and 1.d4 has its great values. The same with Nf3 and c4.

Do it like Fischer: Play 1. e4 for your whole life - and then, when you're challenging the world champion, you suddenly switch to 1. c4 confusing your opponent. xD