If i had to be locked up with one of them i would always go for Guo Fu. At the very least she has some rationality and is not sadistic to the extreme. While GF is rash and bratty she does have some conscience and knows when she does wrong even if she is loathe to admit it.
Ah Zhi strikes fear in me, she would torture you just for kicks.
Even if both had be brought up in the same environment i think Ah Zhi will still be pretty evil.