Orbitz is owned by Expedia now as is Travelocity. If you go to Orbitz (or Travelocity) you'll see that those sites look almost identical to Expedia. Basically they have a slightly different design but have the same "engine" powering them and generally the same (or very similar) pricing.

So to answer your question, Orbitz and Travelocity are not any more or less relevant than Expedia. Note that Expedia also owns CarRentals.com, Hotwire, Hotels.com and a host of other sites.

Expedia is one of the largest OTAs (online travel agencies) in the world, with the other being Priceline (now officially known as "Booking Group").

Priceline owns Booking.com, Agoda.com, Rentalcars.com, Kayak, etc. Confused yet?

To answer the gist of your original question, it pays to shop around for car rentals as prices can vary. Avoid rates that require you to pay up-front and don't allow for free cancellation as rates often drop as the pickup date approaches. AutoSlash offers a free tracking service to let you know when this happens so you can re-book and save. AutoSlash also can provide you with a discounted rate quote taking into account any discounts you qualify for like AAA. Costco, USAA, etc.