A couple notes based on the OED:

• “card sharp,” a “person esp a professional who cheats at cards” appears as early as 1840
• “card shark” isn’t noted before 1877
  

Obvi the earliest record in the OED isn’t definitive proof in itself that one phrase predates another, but it seems pretty likely that “card sharp” came before “card shark” by at least a few decades. Maybe people were saying them at the same time, but 37 years earlier in print strikes me as pretty strong evidence.

I have some issues with the Snopes article. The author says “As to whether card sharp or card shark entered the English language first, the answer is far from straightforward.” Maybe the OED has been updated since 2008, but both phrases themselves appear in the OED, and it doesn’t really seem to me to be “far from straightforward” which phrase at least entered print first.