Elvis and Johnny Cash were early rockabilly artists. Both began their recording careers in the 1950s with Sun studios (run by Sam Philips). Evis' recording contract was sold to RCA and he then morphed into more of a rock and rolls star. Heartbreak Hotel was his first "pop" no 1 - its very bluesy. Johnny Cash wanted to sing gospel originally but was turned down by Sam Philips. He re auditioned and went into country instead. Cash, Elvis, Jerry Lee Lewis and Carl perkins all went on tour together in 1956 and were known as the Million dollar quartet. I like the rockabilly style. Its quite raw and gutsy, with strong lyrics.
By contrast the Nashville style of the mid 1950s was more commercial. It had more instrumentation, and more backing vocals - Nashville attracted lots of session musicians. It was smooth and, in my view a bit bland and safe. But it sold. Jim Reeves and Don Gibson were important artists.
The Nashville style led to more sophisticated recording techniques - an art in itself. This has influenced recording today.