woah and whoa are two different things. Yeah, they're nearly the same noise, and they both kind of have a connotation of "hold on a second," but they aren't the same pseudoword.
Whoa:
1. Sound to stop a horse
2. Wait-wait-wait, what did you just say?
Woah:
1. Dude, you just blew my mind.
2. I'm Screech from Saved By the Bell.