Nasus scales much better into late game than Darius or Renekton. The later two are lane bullies that are very likely to get kills in lanes or bully them into Bing back a lot and totally deny the other champion. However outside of lane, all they can really be are tanks with OK dps. You can get hella kited for going after the carries or just focused down by everyone in team fights, so your only jobs can be the tank that peels for the ADC, or maybe a splitpusher.
Nasus meanwhile is the opposite of a lane bully that falls off late game. He will passively farm and there's nothing you can do to make him leave thanks to his built in sustain and his targeted Wither. Farm his Q up for a half-hour, build mostly tank items and some damage (maybe a Triforce or Bloodthirster, not that experienced with Nasus building) and you're an unstoppable tank with HUGE damage. It is just as possible to become as "unkillable" with Nasus as it is with Cho-Gath. He's a late game monster.
So basically when deciding between Renekton/Darius or Nasus, you're basically asking, do I want early/mid or late game? --- Funky Strong "Date vuelta. Veras el color rojo cuando termine contigo..." - Vega