Rupes Bigfoot LHR15 Mark III vs Rupes Bigfoot LHR21 Mark III Polisher 

Are you trying to decide between the Rupes LHR15 and LHR21 Mark III Polishers?  Both are fantastic polishing machines and in a perfect world, I would have one of each in my detailing arsenal.  If you need to decide on just one we have a breakdown of what each machine does best.  
Both the Rupes LHR15 and LHR21 Mark III polishers are dual action, random orbital machines.  The LHR15 features a 15mm orbit combined with a 5" backing plate.  The Rupes LHR21 Mark III features a 21mm orbit with a 6" backing plate. 
The LHR15 is a great machine for polishing and correcting paint.  The 15mm large orbit runs at a higher speed on a smaller 5-6" pad.  When you decrease pad size, it increases the cutting power.  The LHR21 works best on larger vehicles or in a body shop/panel off setting.  It accomplishes cutting and polishing with a larger orbit than the 15mm unit and a lower speed.  In terms of cutting and finishing speed, both machines are nearly identical.  
The Rupes LHR15 and LHR21 Mark III polishers have many of the same convenience features.  They both have dual rubber bump stops on the back so you can set the unit down without it tipping over or sliding.  The speed dial is now larger, easier to adjust, and is protected by the rear bump stop. The new polishers also feature a progressive trigger that allows for adjusting the speed of the unit on the fly.  This helps when priming pads, working on complex body curves and throttling back when working edges.  
In summary, the Rupes LHR15 and LHR21 Mark III polishers are both great units.  The LHR15 is the most versatile unit to use on smaller panels and areas.  The LHR21 advantage comes when working on larger work area panels.  Both correct and finish well.  Some detailers have converted the LHR21 to a 5" backing plate to gain more cutting power, but some have found vibration is a negative side effect with the smaller backing plate.  The machine is engineered and balanced to work with the 6".  We do sell the Rupes 5" backing plates if you wish to explore this option.  If you have any other questions about these polishers, please contact us.  Happy polishing!