So after further research this was a reply I got from one the experts at etrailer. These guy's are pretty sharp and I trust their advice.
"Dimensionally the 1,200 lb Reese spring bars # RP58358
and the 1,500 lb spring bars of the # RP66130
will be the same. This is intentional so that the rest of the system can work on as many different setups as possible.
The difference is the strength of the spring steel used in the bars. The 1,500 lb bars will be considerably stiffer. To test this you can try using either spring bar in your setup to see if you feel a difference. The 1,500 lb bar should have a much harsher ride quality."
Retired Fire Dept Battalion Chief
2016 Ever-Lite 232RBS
2012 F150 FX4 W/Max tow & Ecoboost
Getting old ain't for wimps!!!