Try to go for positive air pressure in the case or at least balanced, which means more air going in than out. This stops dust collecting inside the case to a certain extent. For intakes, try to buy filters for the fans so less dust gets in. You should also try to follow thermal dynamics, where heat rises. But there seem to be more exhaust than intake for this case, I would go with the second set up, but turn the rear fan to intake. And put filters at all intakes.
isn't sucking cool air from the outside better?
The second picture (Airflow2.jpg) is better. Cool air would be taken in from the front of the case (and sometimes the sidepanel) and vented out from the back and the top of the case. The reason why you should have the top venting is because heat rises, as the heat rises if your top fans are pushing in air what you are doing is pushing the hot air back into the case not allowing it to escape.
If they are venting out air, they would pull the hot air out and the cool air from the front and side panel will go over the components. The reason for the rear case fan is to exhaust the hot air being vented by the cpu hsf.
This is the most simple form of air flow in a case. People do use other methods such as positive air flow (where all possible fans are pushing air into the case with non venting out and negative airflow (where all fans are pulling air from inside the case and non pushing in). However I believe the method shown in the second picture is the best.
Also take into account that the air pushed into the case is from the room its in, so if you have a well ventilated room your pc will have lower temps, whereas if your room is like a box with low ventilation then your room will heat up due to your pc's temp and the warm air will keep on rising.
With mine when the cold air is being drawn from the back the cpu is lower temps up until around 3 hours where it starts rising dramatically
Also I now have the airflow like diagram 1 and it is much cooler than before (on everything - RAM, CPU, inside the Case - EVEN THE HARD DRIVES)
It's just a mystery how it's cooler with the fans setup like in diagram 1!