Splash guards or Muds guards call them what ever but the ones from Parts Cheap are nice and serve their purpose well. My lower fenders and bumper have stayed much cleaner.

 

I like the mudflaps that MarkJ has "on his car," if you know what I mean...

 

Splashguards guard against splashes, but don't flap.
Mudflaps belong on big rigs, not on a Mustang.
Go with the guards.

 

After driving my new Mustang for a couple of weeks and seeing all the crap the rear tires kick up, I installed the factory mud flaps (splash guards). Same as the ones from the website vendor. I was going to order from them, with shipping they are $49.66. Got the same ones at the local dealer for $43.64.
If you have a short screwdriver you can avoid removing the rear tires to install. Takes just a few minutes ... looks good.