It's between three drivers, Barichello, Sutil and Senna.
Williams needs money BADLY, so it's rational to assume that they will have a third driver for FP1 to rake in some extra cash.
Out of those three guys, Senna is probably the only one willing to take that role.
Therefore, in order to maximize their profits, Williams won't take him as a main driver because then they won't have a third driver (unless they find someone completely different, such as Bottas).
After that, it's between Sutil and Barrichello. Sutil probably got a little bit more cash and a future, Barichello got more experience and better coordination with the engineers/mechanics.
It's a 50/50 thing, there's really no way to know. Personally, I'd pick Sutil.