The real difference
1.e4 opens a diagonal for the bishop and the queen, so pieces come out fast and contact happens early. Games get sharp before either side has finished developing, and the player who calculates better tends to win. 1.d4 supports the centre pawn with the queen from the start, which makes the structure more stable — the centre often stays locked for a long time, and the game is decided by plans and manoeuvres rather than a tactical collision on move twelve.
Put crudely: 1.e4 asks how well you calculate, 1.d4 asks how well you plan. Both are learnable. Neither is the mark of a better player.
The theory burden is asymmetric
This is the practical argument, and it usually decides the question. 1.e4 has to be ready for the Sicilian Defense, which is not one opening but a dozen with almost nothing in common — the Najdorf, the Dragon and the Taimanov require separate preparation. Then there is the French, the Caro-Kann, and 1...e5 itself.
1.d4 concentrates. A large share of Black's serious replies transpose into a handful of structures around the Queen's Gambit Declined and the Indian defences, so one set of ideas covers more ground. You still need answers to the King's Indian and the Nimzo-Indian, but they share a vocabulary in a way the anti-Sicilians never will.
If your study time is measured in hours per month rather than per week, that asymmetry matters more than any stylistic preference.
How to decide
Play twenty games with each and look at how you lost, not how you won. If your 1.e4 losses are tactical oversights in complications you enjoyed, stay there and work on calculation. If they are slow strategic drifts where you never had a plan, 1.d4 is not going to fix that — but it will at least let you practise plan-making in positions that reward it.
And note that this is not a permanent decision. Most players switch at least once. The opening knowledge is not wasted, because the understanding of the structures transfers.
Frequently Asked Questions
Is 1.e4 better for attacking players?
Mostly yes, but it is a tendency rather than a rule. 1.e4 produces open positions where attacks arrive early and directly. 1.d4 attacks exist too — the King's Indian Sämisch and the sharp Queen's Gambit lines can get extremely violent — they simply take longer to build, and they require you to have prepared the position for them first.
Which is better against a stronger opponent?
1.e4, marginally, because sharp positions are harder to steer and a stronger player has fewer chances to slowly outplay you. The trade-off is that sharp positions are also where superior calculation shows up most clearly, so the advantage is smaller than it sounds.
Can I play both?
You can, but not early. Two first moves means two full repertoires and half the reps in each. Most players get more out of knowing one deeply, then adding the other once the first has stopped producing surprises.