design - Functional difference between various astable 555 circuits

Looking through some 555 square-wave-generator circuits (possibly with variable duty cycle for 4-wire PWM fan control) I've found three different circuits:

• From the Wikipedia 555 article, a "standard 555 astable circuit":

• From DPRG, a "simple PWM circuit":

Functionally, these circuits all seem to do the same thing, but seem to be vastly different in design, particularly the last two (the Wikipedia circuit lacks variable duty cycle, though the article briefly mentions it); short of simulating them, is there a way to determine which is "better" or are they equally valid?