* Periods of Revolution: Saturn and Neptune have much longer periods of revolution (time to orbit the Sun) than Mercury and Mars. This is because they are farther away from the Sun.

* Orbital Velocities: Planets farther from the Sun actually have *slower* orbital velocities. This is because the gravitational pull of the Sun is weaker at greater distances.

* Distances from the Sun: Mercury and Mars are much closer to the Sun than Saturn and Neptune. This is the key difference.

* Equatorial Diameters: While Saturn and Neptune are larger planets, it's not a guarantee that Mercury and Mars have greater equatorial diameters. In this case, Mercury and Mars are both smaller than Saturn and Neptune.

Therefore, the correct answer is (3) distances from the Sun.