Reasons for the stability of SF6:

* Larger size of sulfur: Sulfur is a larger atom than oxygen. This larger size allows for better distribution of the electron density around the central sulfur atom, reducing electron-electron repulsion and increasing stability.

* Higher electronegativity of fluorine: Fluorine is the most electronegative element, and its strong electronegativity helps stabilize the S-F bonds in SF6. The high electronegativity difference between sulfur and fluorine results in a more polar bond, which contributes to the overall stability.

* Inertness of fluorine: Fluorine is a highly unreactive element, and its presence in SF6 further contributes to its stability.

* Octahedral geometry: SF6 has a perfect octahedral geometry, which is a highly symmetrical and stable arrangement of atoms.

Reasons for the instability of OF6:

* Smaller size of oxygen: Oxygen is a smaller atom than sulfur, leading to greater electron-electron repulsion and less stable bonds.

* Lower electronegativity difference: The electronegativity difference between oxygen and fluorine is smaller than that between sulfur and fluorine, resulting in less polar O-F bonds and lower stability.

* Reactivity of oxygen: Oxygen is more reactive than fluorine, making OF6 more susceptible to decomposition.

Note: OF6 is a highly unstable and explosive compound that exists only under specific conditions. It is difficult to synthesize and has limited applications.