Ask two salsa dancers which timing is 'correct' and you may not get out of the room before midnight. On1 and On2 are the two dominant ways of stepping to the same music, and dancers can be surprisingly tribal about them — but the honest answer is that they are dialects of the same language, not rival tongues.
On1, the style most associated with Los Angeles, breaks forward on the first beat of the bar. It is bright, driven and punchy, and because the break lands with the most obvious accent in the music, it tends to be quicker to pick up for beginners. The energy is up front and the dance feels propulsive.
On2, often called New York or mambo timing, breaks on the second beat, tying the dance more closely to the salsa rhythm section — specifically the tumbao of the conga. Dancers who love On2 talk about feeling inside the music rather than on top of it; the pauses fall differently and the whole dance sits deeper in the groove.
Neither is harder in any absolute sense, though switching between them after years in one can feel like writing with your other hand. What matters far more than the number is that you can actually hear where the beat is. A dancer with rock-solid timing On1 is a joy; a dancer chasing On2 for status while losing the beat is not.
If you are starting out, learn whichever your local scene dances most — you will get more practice and more patient partners. The timing wars are mostly a spectator sport for people who have forgotten that the point was always to dance to the song.