Open Case vs Air Flow

Your case only has a front and a rear fan which isnt going to move much air through it, something like an Antec 900 or a HAF 922 will move a lot more air so the air in the case stays cooler which cools everything else down. In a case with a good fan setup having the side panel on will improve temperatures because the air gets to travel quickly having the side panel off makes all the air move slower but if you air isnt moving quickly to start with an injection of cool air mixing through the side can help.

Usually open-case is worse, because when you have it closed the air will at least flow. On the other hand, if it is open, the ambient air will just surround it and it won't really do much.

An open case is not going to have directed airflow.. A good directed airflow setup ensures optimum heat transfer off your components which is not what a open case panel can provide.. The need for well good airflow increases as you cramp in more components.. Coming to recommendations, CM Storm Sniper if you are searching for a mid tower and Lian-Li PC A70F if you are looking for a full tower..

the open side case have disadvantages as well its good option your pc may store the room dust

Best thing about an open case: if your temps are better with the side off, it tells you that you either need to improve the airflow through the case or get a better case.