Whether offline or online is harder depends on how you look at it; in offline, for example, you have the right to make a mistake, whereas in online you don't, so both Tournaments, in my view, are quite difficult and prestigious.

And that's why I wrote that we need to separate the wheat from the chaff. In on-line, usually two people play (with more players, the situation is even worse). There, you can make an accidental mistake, the opponent might be inherently weaker, map location, resources, race and all such things can affect the outcome, which may not show the player's real strength.
But in OFF-line EVERYONE (dozens) plays THE SAME MAP and has THE SAME starting conditions. Each of them has the right to make a mistake and replay several times, which ultimately shows the REAL strength of a player in the game Heroes.
To some extent, ON-Line also shows a player's strength. Naturally, whoever is more experienced and reactive has a better chance of winning.