Diamond vs the Box formation
A disclaimer first that I put at the beginning of all my tactical advice posts is that everything I advise is always done to play football with quality not playing percentages or just to win. It is done to provide the best base for your team to play real football.
In 5 a side really there are 3 basic formations but only 2 I really advise as one is a very slight variation on the other that is not over advantageous. So the 2 main ways to play are either with a 2 2 formation or a 1 2 1 diamond. I will outline the reasons for each one and what I recommend below:
The 1 2 1 diamond
This personally is the formation I like to start with as there are more clearly defined positions and it lends itself much better to the progression to larger sided games. The focus is on maintaining that diamond around the pitch when in possession of the ball allowing clearly defined avenues of support in 3 directions. Also when the ball is played into the forward player or defender there should be a clear area to switch play (the opposite wide player), allowing us to transfer the ball from an area of high pressure to an area of low pressure easily. We should always have forward options and a backwards pass with the players looking to maintain their space and move within it.
The key aspects of this are:
Maintaining the diamond to allow 3 passing options at all times
Ensuring the defensive player always gives the option of a backwards pass to help maintain possession
Probably having your most intelligent passer at the back
Ensuring your striker holds their position up front without becoming detached from play
Making sure your midfielders understand they have to work in both directions to attack and defend ensuring they make this transition quickly
Advantages
1. If you do it right it makes it very easy to play good passing football as it naturally uses the maximum space on the football pitch
2. Switching play; one of the most effective ways to open teams up as naturally all players will move towards the ball in order to win it back, is very easy in this formation.
3. It starts to build a real idea of positional understanding and the kind of supporting angles that are vital as they progress up to larger sided games.
Disadvantages
1. It can sometimes be a bit too rigid in terms of positional rotation with younger players this level of understanding needed to effectively change and cover positions is not always there.
2. The need to hold certain positions to make the formation effective can be a challenge to players especially if you like to rotate positions to develop their football knowledge as there are different demands for each position
3. It leaves you quite prone to the counter attack as you will generally attack with 3 players moving forward with the ball and one slightly behind. The midfielders will have to recover very quickly and if the defender gets caught out they can be little to no cover.
4. As mentioned above it gives the defender very little room for error.
5. If you don't pass the ball backwards enough to retain possession or you don't get the ball forward quickly enough the defender and striker can get quite bored as they are asked to be very disciplined in these positions.
The 2 2 box formation
This formation has 2 defenders and 2 attackers set in a square or box. Position wise it lends itself more to a progression to a 4 4 2 with the CM and ST. In this situation we should also have 2 people attacking and at least 2 defending every time. It also allows a quick counter as you will always have 2 people in position to attack. You should be able to combine in the forward positions and have an easier pass at the back to keep possession. The 2 up front and at the back need to shuffle across with possession to keep adequate lines of support around the ball.
The key aspects of this formation:
Maintaining at least 1 person forward and in defence at all times. Trying to keep 2 forward and 2 back and even with rotation and movement always having at least 1 there.
There is a level of flexibility within the 2 forward positions allowing some clever combination and freedom of movement.
The back 2 need to work as a team when one shuffles across to support the other needs to offer a backwards or sideways passing option. They may also need to ensure that one picks up the opposition player while the other supports the player on the ball.
Advantages:
1. You have more defensive security as you are almost guaranteed to have 2 players back at all times.
2. It allows you the option of counter attacking quickly as you should always have 2 passing options going forward.
3. You should always have support in attack as there should be 2 attackers at all times so more options.
4. There is more positional flexibility as in this formation there is a lot of security so there is license for the attacking players to move and rotation to provide width, to create space and to open up the opposition.
5. The main advantage I see is that it is very simple and there isn't as much emphasis on getting positioning totally correct all the time as there is in the diamond formation. This then means that new teams can adopt this formation very easily.
Disadvantages:
1. Roles aren't clearly defined so it allows players to be headless chickens. This can be helpful to young players but also it stops their tactical development and understanding of the game.
2. As above without a clear structure to where each forward player needs to be; although there is nothing stopping you giving more instructions that may help in this regard players can congest around the ball. This could massively reduce the quality of your play or the ability of players to make passes.
3. There isn't natural width on both sides and if the players pull wide they either vacate the middle area or basically play in a diamond position if 2 pull wide and 2 stay in the central areas.
4. Your team could end up being basically 2 teams in one. 2 players who only attack and 2 who only defend.
5. Although obviously it is open to rotation it takes out the natural midfield position a key position that they will have to understand and develop as they get older.
In conclusion I feel both formations have their merits and you wouldn't be wrong to choose either. I have always stuck to the diamond formation but often see teams beat me when we get constantly caught on the counter. Seeing this sometimes makes me wonder but the developmental advantages of increased positional understanding, the possibilities of being able to play good football so much easier (as there are clear areas to switch play to), and the fact it is so easily relatable to formations in 7, 9 and 11 a side makes it a much better formation to develop your team.
However you will almost definitely have more success in the box formation at the beginning stages of your teams as it provides much more security and room for error. It allows you to compensate for technical weaknesses and may be a good formation to start with while your team builds up their ability to play in a more expansive style.