Seeing as this gets asked alot, thought I would air my views and personal experience from another post where I previously stated this.
Why I purchased the 15 over the 21 was:
1:Smoothness, claimed to be the smoothest single head DA in the world and I truly believe that claim.
2:Ability to stall less on curves over the 21,naturally as a result of the shorter stroke.
3:Can fit into tighter spots with the shorter stroke over the 21.
4:Less aggressive on pads than the 21 makes it more economical long term.
5:Suited and designed for the 5" backing plate and 6" pads without compromising smoothness and machines longevity.
6:Rupes 21 designed for bodyshops and large flat panels. Rupes 15 designed for the Detailer and even says so on the polisher. Every car I've detailed has curves opposed to vast areas of flat panels.
7:Throw/Stroke length is just perfect for almost all needs and requirements.
Why I purchased the 15 over the 21 was:
1:Smoothness, claimed to be the smoothest single head DA in the world and I truly believe that claim.
2:Ability to stall less on curves over the 21,naturally as a result of the shorter stroke.
3:Can fit into tighter spots with the shorter stroke over the 21.
4:Less aggressive on pads than the 21 makes it more economical long term.
5:Suited and designed for the 5" backing plate and 6" pads without compromising smoothness and machines longevity.
6:Rupes 21 designed for bodyshops and large flat panels. Rupes 15 designed for the Detailer and even says so on the polisher. Every car I've detailed has curves opposed to vast areas of flat panels.
7:Throw/Stroke length is just perfect for almost all needs and requirements.