What are the differences in character and cultural background between the first narrator and Simon Wheeler?
The first narrator is seemingly well-educated and as pretentious as it gets. His language and his condescending tone and description of Simon Wheeler give the impression that he (narrator) is upper class, privileged, self-righteous, and basically has an undeserved sense of entitlement. This is Twain mocking the stereotypical pompous yuppie snob (probably of the northeastern variety).
Simon Wheeler is described, and his story is retold to us, by the narrator. So, the Simon Wheeler we get, and the way in which the 'tall tale' is told is through the narrator. Wheeler is described by the narrator as a simple country yokel. This initially implies that Wheeler is rural, uneducated, and barbaric. However, his tale has clues that while he may be southern, rural and a country folk type, he is the one who is mocking the narrator. Not the other way around. In the tall tale, Wheeler mocks the clergy (knowing the narrator will find this offensive) and mocks politicians by making historical references with the names Andrew Jackson and Daniel Webster. Wheeler may even be intentionally playing up the role of the simplistic, uncomplicated hayseed to mock the narrator without him even knowing it.
This is a trick within a trick. We think Twain is the narrator initially; or maybe Jim Smiley. But it is Wheeler, the teller of the tale that is Twain's voice.