Why space vacuum does not suck out our atmosphere?
The concept of space sucking out our atmosphere is a common misconception. In reality, space itself doesn't suck or have the ability to pull our atmosphere away. Instead, it's the result of several key factors that protect our atmosphere:

1. **Gravity**: The primary reason our atmosphere stays with Earth is gravity. Earth's gravitational pull is what keeps the gases of our atmosphere close to the planet's surface. Gravity is a fundamental force of attraction that pulls everything with mass toward each other. The strength of Earth's gravity is strong enough to hold our atmosphere in place.

2. **Escape Velocity**: To escape Earth's gravitational pull and enter space, an object must reach a certain speed called the escape velocity. This speed is about 11.2 kilometers per second (about 25,020 miles per hour) at the surface of Earth. Gas molecules in the atmosphere don't naturally achieve this velocity, so they remain bound to Earth.

3. **Pressure Gradient**: The atmosphere is composed of layers, and there is a pressure gradient from the surface up to space. As you go higher in the atmosphere, the pressure decreases. This gradient creates a balance that keeps the atmosphere close to Earth.

While Earth does lose some gases to space over extremely long periods of time, it's a slow process and doesn't pose a significant threat to our atmosphere. Our planet's gravity is the dominant force that keeps our atmosphere intact and prevents it from being sucked out into the vacuum of space.
Which of these will have the greater effect on retaining our atmosphere?
All the factors are interrealted. Gravity results in the concept of escape velocity and pressure gradients. So fundamentally, it is gravity that keeps our atmosphere bound to the planet. However, do note that some studies show that other factors like a planet's magnetic field also come into play. Without the magnetic field, the solar flares and other cosmic energies can easily strip away an atmosphere from the planet.