To someone beginning a journey we say, “Leich l’shalom”, “Go toward peace”. The Talmud finds biblical endorsement for this phrase: Jethro told Moses “Leich l’shalom” when Moses set off for Egypt. And we all know that his trip was a success. On the other hand, Kind David told his son Absalom “Leich b’shalom” (“Go in peace”) and he ended up suspended from a tree and shot by an arrow.
The Torah Temimah explains that “Leich l’shalom” is not merely a wish for a bon voyage. It actually refers to the striving for ever greater spiritual perfection. Shalom derives from the word shalem, meaning whole or perfect. Leich l’shalom is a wish for you to travel toward the next level of shleimut or perfection.
On the other hand, Leich b’shalom implies that you will stay on the same level at which you began. The Talmud says that “Go in peace” is only really an appropriate wish to one who is dead.