What is the advantage of playing non-lethally?
Solution 1:
According to the Watch_Dogs wiki:
When performed stealthily, a takedown won't alert anyone and will reward Aiden with bonus experience. Using takedowns on criminals will increase Aiden's reputation as well.
I'm not sure how much more experience you get though.
If they don't know where you are that gives you a tactical advantage enabling you more room to maneuver around the area and plant traps or hack things. This in turn can make the game significantly easier. However you have the freedom to go "guns blazing" anytime you wish.