Uber / Lyft for sure! Went to Red Rock from Excalibur with a friend and paid $18 one way so $9 per person (this included a $5 discount). What a joke!
You might also want to check my website
http://uberinlasvegas.com/ so you know where you need to go when you ordering an Uber or Lyft, most hotels have a special Uber/Lyft pickup spot.
Enjoy your stay and go to Red Rock, it's great (especially the buffet hehe)!