Answer
$H_3CSH$ is the better structure because the formal charge on $C$ and $S$ is zero in this structure.
Work Step by Step
We know,
formal charge = no. of valence electrons - (no. of nonbonding electrons + 1/2 no. of shared electrons)
In $H_3CSH$,
Formal charge on carbon = 4 - (0+ 1/2*8) = 0
Formal charge on sulfur = 6 - (4+ 1/2*4) = 0
In $H_3SCH$,
Formal charge on carbon = 4 - (4+ 1/2*4) = -2
Formal charge on sulfur = 6 - (0+ 1/2*8) = 2
$H_3CSH$ is the better structure because the formal charge on $C$ and $S$ is zero in this structure.
