Marino's first five years in the league changed football, which is why he's in the Hall of Fame. The rules changes in 1978 opened up the game and Marino single-handedly elevated the NFL passing game in the 1980's.
After those five years, though, the league adjusted to pass-heavy offenses and Marino had far less of an impact.
The facts are Romo throws for more TD's and yards and fewer ints per throw than Marino against competition better prepared to defend the pass. I don't GAS that Marino is an NFL legend.
The comparison is valid, except for physical stature. Marino had a stronger arm (though not as strong as Elway or Roethlisberger) and was far more durable, Romo actually has a quicker release (which is mind boggling considering Marino set the standard for quick releases) and is more elusive.
Marino failed because he refused to be part of an offense that ran the football. I believe Romo wants to run the ball if it works, but it never seems to when he needs it most.
Because he started as a rookie, Marino played 17 seasons as a primary starter. No way in Hell can Romo equal that.