The comparison is skewed depending on what point in history you are comparing from. In 1957 you are comparing a three year old body shell (Chevy) to a brand new one in two sizes (Ford) at a time when ALL NEW and LONGER, LOWER, WIDER were shouted out as a very big deal. The larger size Fairlane 500 was marketed as a reach up into the lower medium priced field and in fact there is a vintage sales film on You Tube comparing it (and compellingly so) to a base 1957 Pontiac. So from a marketing perspective when new the 1957 Ford really moved the needle and the Chevy was just another refresh. A pretty good refresh, but still bad news in the 1957 market.
But as a used car, say, 5 years later in 1962, the Chevy looked pretty good. New cars had returned to more subtle, restrained lines and fins and bold two tones were passe. So now a 1957 Fairlane 500 or Plymouth Belvedere looked like an old 1950s car and the Chevy that was a bit behind in 1957 now looked much less dated than the others. The basic 1955-57 Chevy body was comfortable and a convenient size compared to the bigger, lower cars that came later. It was also of better build quality and rust resistance. I contend THIS point is where the 1957 Chevy began to become an icon, as a solid and serviceable used car. Of course IMO a 1957 Pontiac is better but it was not destined to be such an icon, too bad for me.
The engines mentioned by our own capngrog were a factor too. Magazines of the day make me think to the new car buyer in 1955-57 the Ford Y Block was considered just as good as anything. But by 1962 the small block Chevy was recognized as more serviceable, especially by the teenage owners that would covet the 1957s later in life. As Grog points out by then the new Ford small block was coming out and that pretty much admitted the Y Block had run its course, Todd C
PS--I agree with others that the dash of the Chevy was odd looking and the Ford headlamps were the weak point of an otherwise very good design