[Mock8800]

Leveling up gives

Increased success rate at crimes
You perform better in PvP
Better trains in the gym
More AP to do stuff
Increases the max time you can attempt in jail busts
Increases the success chance of jail busts.
Allows you to take classes that require more energy.

The only benefit to staying at a lower level is you have the possibility to get more gang points from an gang attack.