Either will produce the exact same quality of audio.
You only need one.
If your LG has a disc player built in then you will need the HDMI cable anyway. ARC was designed to eliminate the need for an additional cable to get sound from the TV to the HTS.
The other potential advantage of using HDMI-ARC is that it may allow for some control of the HTS from the TV such as turning on the TV will turn on the LG.
Both the TV and HTS have to support ARC for it to work. If usually has to be turned on in the TV menu. Sometimes you also have to turn the TV speakers off to get it working. Check the TV and HTS manuals.