The terminology itself generally derives from the location at which it's spoken. The western half-or-so of the country, and the southern states, calls it soda, the midwest area (Around where I live, Minnesota, the Dakotas, reaching up to about New York) call it pop, and further northeast, in Maine, Vermont, and New Hampshire, they call it tonic.
As for me, I tend to call it by what it is. As in, "I'd like a Pepsi, please."
But if I had to choose, I'd probably say pop, because that's what my whole family says.