This is just plain simple. Robbing will give you a lot of big cash and Fighting will give you big experience.
Here are the obvious diffrence of the two:
Rob
1. It will gave you some big cash depends of the property your robbing. If you succeed to robbed a Casino you can have more than a million. Good enough rather than having 20x fight.
2. A little chance to get good experience.
3. No chance to have rare loot.
4. Since theres a lot of people robbing others property you have less chance to be on the hit-list.
5. You have a bigger chance to get killed because properties has of big damage in returns.
FIght
1. The Cash it can give you will depend your safe-stocker crew and how rich is your foe, this can be like 100k or hgiher for a success attack.
2. You have a chance to have a big Experience in fight but that not much by the help of your mafia and it depends also your Master mind-crew.
3. Has a chance to pick some rare loot .
4. Prone to be in the hitlist.
5. You can still resist the damaged from your foe that will keep you alive.