$\overline{SAT}$ vs. $UNSAT$, Is it the same?
They are not precisely the same language. For instance, $\overline{SAT}$ has words that are not even formulas. $UNSAT$ on the other hand, has only formulas that are not satisfiable.
In terms of complexity measures - the two languages are equivalent, since we can check in polynomial time whether a given input word is a formula or not and therefore we have a poly-time reduction from $\overline{SAT}$ to $UNSAT$ and vice versa.