Sun, Do you have a Yeoman who takes care of your pay? They would be the best person to explain the differences, pros and cons. A lot of it depends on your situation and information. I encourage you to get your information from your Yeoman. And, if you are someplace like a station where you don't have a Yeoman, ask your XPO who the SPO Yeoman is and find out if you can talk to him/her.