Some petrol stations are open 24hrs. This makes them an easy target at night when all the streets are clear and there are very few witnesses. The best time to rob them (and Im thinking this is where the OP is going), is when they change over and the keys, etc are handed over.
If you want to rob a bank, generally you would need to do this during the day time. This will require big time planning and will also require many associates. The more people you have in on it, the more chance there is of somebody doing/saying something wrong and giving the game away. You will have to deal with the customers who in the bank, holding them hostage (not good, if you get caught), as well as subdue/convince the members of staff (who will be in high numbers, compared to a petrol station guy, doing the nightshift on his own, who is more likely to cooperate as he is on his own).
The money in a petrol station might not be much (compared to a bank), but there is less chance of you being identified and getting a knock on your door 10 days after the robbery.
The OP has gone quiet now. Perhaps he felt that he was giving us too much information.
Keep an eye out in the newspapers for someone robbing a petrol pump. He may even get caught and claim a guy called 'sunama', on a computer forum, helped him mastermind the robbery. Dont believe a word he says.