I thought its self evident for this game that characters lvl up on different times bevore reading this topic, because we stay close to baldurs gate and icewind dale and neverwinternights already proofed it, that it sucks.
It can be very tactical when u have a wizard who is 2 lvl behind and u have to take some extra care of him. When all characters lvl same time, in my opinion , it can be annoying and interrupt the game when u have to spend some 15-20 min on leveling cause u want to do everything right and forced to read every description for every skill for that reason and later u have to wait ages, just to sit there again 20 min for lvl up.
Divide it over a longer time of the game is more tactical and also provides u with a constant feeling of success.