Their signing bonuses are similar but their base salaries are no way comparable. Rookie salaries are much much more cap friendly.
Also, with the high dollar FA, you dont always know what you are getting.
See Hugh Douglas, Clinton Portis, Jeremiah Trotter, Bobby Taylor just to name a few of my FAV overpriced FA flops in the past 1-2 years.
You also dont get the mileage on a FA RB as you would with a rookie RB. They both are risks but you just need to weigh the pros and cons and see how they perform.