I don't live in the Columbia area, so this information may not be accurate. You'll want to verify for yourself.
In general, monitoring a single site at a time is an advantage. It essentially speeds up scanning because the scanner is not required to jump and retune to more than one control channel. If you're simultaneously scanning a trunked system and conventional channels from the same radio, this can be an advantage.
I've never heard voice traffic on the Columbia site. Only an active control channel. If you monitor that site, you may not hear anything.
If you're near enough to monitor Harbison, and you struggle with simulcast distorted reception from the Richland Simulcast site, then monitoring Harbison may be an advantage. I'm not sure how many Richland County talkgroups are carried on Harbison on a 24x7 basis - perhaps not talkgroups for LE regions or agencies toward the south end of the county. I believe Harbison carries a mix of Richland and Lexington talkgroups. If you're trying to listen to talkgroups from both counties, monitoring Harbison might be an advantage.