The main advantage for using the Mozilla-provided, Marionette-based Geckodriver solution is that it works for versions of Firefox 48 and higher. The legacy driver provided and maintained by the Selenium project doesn’t work for Firefox 48 or higher, and will never work for those versions of Firefox.
The legacy driver is implemented as a Firefox extension. This extension is installed in the profile used by the driver when WebDriver launches Firefox. Firefox 48 introduced two new features that disabled this browser extension. The first is the so-called “electrolysis” feature, or multiprocess Firefox. Electrolysis changes the way extensions have to deal with the browser in ways the Selenium team has not taken the time to fully understand.
The second, more important requirement is that all browser extensions must be signed by Mozilla before the browser will allow them to load. This latter feature has been in Firefox for several versions, but beginning with 48, it can no longer be disabled. The WebDriver browser extension introduces several valid security concerns for the Firefox browser, and as such, will not be signed by Mozilla’s security team. This, in turn, renders the extension inoperable, and thus Selenium can no longer communicate with Firefox. The Marionette-based solution, being developed and maintained by Mozilla in the first place, is blessed by them for use in automating Firefox, and as such carries a commitment that it will continue to work with future versions moving forward.
So the primary benefit of using Marionette with Firefox 48 and higher is that it will work, whereas other solutions won’t.