The GFDL is mostly intended for books or larger written documents like manuals – it is literally a Free Documentation License. The license has unique provisions like that some sections can be marked as non-modifiable, or that certain attributions have to be shown on the cover of a book. All of this is difficult or impossible to apply to an infographic.
In contrast, The Creative Commons licenses were written with all kinds of creative works in mind, including graphics. For that reason alone, you will likely prefer the CC-BY-SA license. The CC licenses are also much more common and better understood, thanks to their modular structure.
I'd recommend that you include the attribution and CC-BY-SA note within the infographic itself to make it easier for others to share. Infographics often have a section with sources, that would be the ideal place.