550W Plus gold have better internal components, wich provide better power quality (stable) and have additional protections, like overvolting, undervolting and other.
V550W is a better quality PSU.
check those links for full description.
http://www.coolermaster.com/powersupply/enthusiast-v-series/v550/
http://www.coolermaster.com/powersupply/thunder/thunder-500w/