Did the Tutorial really say this? It is not accurate. Losses are higher (for both sides) in an assault. A blitzkrieg is more likely to succeed, can force the defenders to retreat, and is less likely to disorganize the attacking units.
It did. The tutorial said the defenders could choose the type of attack and assault was the preferable option. Defenders, not attackers. Thanks to the previous answer it makes more sense now.
Usually the defender wants to choose assault, but not always. The one point that was not right is that the statement that assault has less losses. This is not true, assuming that a retreat path is available. Assault is likely to have higher losses for both sides. Its advantage for the defenders is that they are more likely to hold ground and to disorganize the attackers. Its disadvantage is that the defenders are more likely to lose units.
If there is no retreat path, then always choose assault as the defenders, since retreating will just get your units killed.
One other comment: WiF is blessed with two completely different combat result tables: the 1d10 table and the 2d10 table. In any given game, one uses one or the other. The 2d10 table is somewhat bloodier. The 1d10 is designed for if you are playing the game without divisions, the 2d10 if you are using divisions. This is stated nowhere, but should be mentioned. Whichever you use, always use the fractional odds optional rule; it saves the players a lot of effort not having to search for that one more factor that gets an odds shift.
I thought I knew how to play this game....