What is the difference between an NVIDIA legacy binary proprietary driver and a binary only proprietary driver?
Basically, the difference between the two drivers is that the legacy drivers are intended for the video cards which are no longer supported by Nvidia in the official driver (thankfully, Nvidia's support is just awesome. You're only out of luck with truly ancient hardware that's like more than 8 years old). The legacy driver is not as well maintained as the newest driver and generally only receives important bugfixes. It will no longer receive new features or optimizations for new games or applications as is the case with the normal driver.
So, it is highly advised to always use the normal drivers if they are available. This way you will have the best experience and support. If you are unsure if your card is still supported by the latest driver, here is a link with a list of supported video cards (click on the tab called "Supported Products"):