Solos work best if you can drill the holes in the leg and chop for the mounting pins on a drill press, and that means getting your bench leg onto the press table. This is impossible on a completed bench. In this case, buy the Retro. Instead of drilling holes for mounting pins you'll fit Retro brackets inside the leg and chop mortise. You can of course use a Retro on a new bench if you'd rather install the brackets instead of the mounting pins. We recommend the Solo for new benches because its quicker and less expensive to install than the Retro.