@paddy dude, ur answer is wrong...here is the correct explanation...i'm damn sure bout it...

the order of Aufbau principle is (energy)

3s < 3p << 4s < 3d

energy of 3d is much larger than that of 3p than also it participates in hybridization...this happens because:

It all depends on the electronegativity of the atom attached to the central atom. In SF6, due to high electronegativity of F, S acquires partial positive charge, and we know when +ve charge comes, the size of atom reduces...because of size reduction 3d orbitals come closer and are able to participate in hybridization.

while Cl isn't that electronegative as F therefore, it is unable to bring the orbitals, hence 3d orbitals don't participate in hybridization and SCl6 doesn't exist...

hope its clear now...