Positive surprise and negative surprise have nothing to do with it. Consider the birthday party guest who says, "Wow! This cake is delicious!" versus the soldier that says, "Wow! That bullet nearly took my head off." Likewise, "Whoa, I think this is the winning lottery ticket!" versus "Whoa, I'm about to be sick."
In fact, the two can occasionally be used interchangeably. Let's invert my prior examples:
Whoa! This cake is delicious! (does not sound right)
Whoa! That bullet nearly took my head off. (fine)
Wow, I think this is the winning lottery ticket! (fine)
Wow, I'm about to be sick. (does not sound right)
Two examples work with either wow or whoa, the other two don't, at least to my ear.
Rather, the difference between the two is simply that "Whoa," in deriving from the command for a horse to stop, is a figurative command to the world and to those around you, to stop (and to think about what has happened.)
To say, "This event was so surprising, so difficult to even comprehend, that I need to stop everything else while I take the time to process the event and attempt to understand the implications" is a mouthful. Instead, "Whoa!" is quite a bit simpler.
The definitions at Merriam-Webster support this:
1: a command (as to a draft animal) to stand still
2: cease or slow a course of action or a line of thought : pause to consider or reconsider —often used to express a strong reaction (as alarm or astonishment)
