I am preparing for a tournament, so which opening should I use or prepare? How can I become strong in a specific opening line?
Tough to tell
Benefits of Both
d4 is very strong controlling two central squares (e5 and c5) e4 controls 1 (d5 since f5 doesn't help develop pieces it's not the best)
e4 opens up the white bishop and queen diagonals (Though queen's is easily blocked), while d4 opens up the bishop, d4 is also benefit by being naturally guarded by the queen.
Analysis of Both
I personally find d4 to be better at gaining control of the center, especially with my favorite white opening, the queen's gambit. Since black can't attack the e pawn black is forced to develop slower since it can't openly attack the center. In this case, d4 is solid.
e4 on the other hand promotes more activity from other pieces by opening up the two diagonals (Though the queen's is almost never used). Yet, since this allows black to fight for the center I have found more exchanges, and I like using e4 for creative openings such as the Ruy Lopez, Rio Gambit, or learning the Sicilian lines seizing space as white while black has to open up space for his pieces to develop.This makes e4 a very good way to open the board, though this opens up many weakness for white often in exchange for black's lack of development.
At high level play, the Sicilian is threatening and at GM-level play e4 has a very low win rate compared to d4 (Refer to Stat's section). It lacks the solid control of the center and e4 promotes a large amount of counter-play, though in my opinion, it doesn't matter at the moment. It's nice to play both and find your own comfort and flexibility.
If you are looking for lines to learn my top 3 for white (in order favorite-least) are: Queen's Gambit, English Opening, and for Black: Sicilian, Nimzo-Indian, and Modern Defense. These are solid and work great at tournament level play.
Chess.com's Opening Explorer (https://www.chess.com/explorer) documents e4 as having (38% wins, 32% ties0, 30% losses) with 879,507 recorded games. d4 is recorded as having (39% wins,34% ties,27% losses) with 693,538 games archived. While this is a fairly close margin the major notable point is the 3% deviation of loss rates, but these ideas aren't imperative as they encompass all levels of play.
The World Championship test Another method to compare these two moves, using actual facts and information instead of general ideas, is to observe the world championships. In a world championship, the players prefer the move they consider more beneficial; the move they believe will offer them more opportunities to achieve victory (especially when playing white). The contenders have, of course, practiced time and again and every defeat signifies a major weakness. What is more, world championships constitute a great study sample simply because there is no other tournament where players demonstrate such seriousness and concentration. Let us examine the results of the modern chess era, meaning the most recent world championships, starting from the year 2000 when Kramnik became world champion.
"In 89 games 1.e4 was used 25 times resulting in: only one win, three losses and 21 draws. In 89 games 1.d4 was used 62 times resulting in: 18 wins, 6 losses and 38 draws. 1.e4 wins: 4% losses: 12% draws: 84% 1.d4 wins: 29% losses: 9% draws: 61%"
At championship level play there is a drastic deviation between the openings, nearly 25%.