According to the spec sheet, the PLS8 is backwards compatible with the PHS8. The PHS8-US covers UMTS bands 850 and 1900, plus the 850 Nd 1900 GSM bands: if your carrier uses either of these (which is most of them I believe) then you'll be fine with the PHS8.
The PLS8-US covers those bands plus the (1700/2100) band for UMTS, the 900 and 1800 bands for GSM, and the 700, 850, (1700/2100), and 1900 bands for LTE. If your carrier is any of these (ie, Wind Mobile in Canada or T-Mobile in the US) then you'll need this "4G" module even for 3G connection.
Summary: The PHS8 can connect to all the same networks as the PLS8, plus some more including LTE networks.