Good Afternoon again,
There is a time and place for each technique. Although I have a different opinion than Cyriacus on the utilization, I certainly agree that neither should be discarded!
For me a Straight Back Kick (Dwi Chagi) is faster and more accurate for quick exchanges in close range as often encountered in sparring, also it keep the target area away from your opponent and leaves your shoulders and body angles to make a quick retreat to put some space between you and your opponent. On the flip side if your opponent likes to retreat a back turning side kick (dwi dollyo yop chagi) puts you in a better position to continue the attack with a follow up kick.
Some may also say that the Side Kick version is faster - Personally, i find it to be the other way around
The Side Kick way can also make it easier to step down, rather than needing to shuffle slightly
In free fighting, and self defense situations where it is a great idea to punch to the face, the back turning side kick puts you in a position to follow through with a good straight punch to the face!
Determining the time and place for an attack is the key ingredient! Train long, train hard, and train often!
TAE-KWON!