I think if both had their best day out, Snake was more damaging. Prior to Snake's injury woes, he had a higher ceiling than Slater too; meaning had he stayed injury free, he'd have finished with more accolades than Slater. Alas, he got injured repeatedly and lost a yard of pace and ended up playing the back-end of his career crippled.
But...the narrative says Slater finished with more tries, more games, more premierships and is generally considered a superior player.
So, I guess there are supporting arguments for both sides. On potential, Snake every day of the week but the books say Slater...