I'd like to see how Slater would go in a side with an average hooker and average halves. Until that happens their respective performances shouldn't even be compared. If Slater was playing fullback in the current Eels side he'd struggle massively IMO because his main strength, his support play, would be wasted because there'd be no-one giving him quality ball or setting him up for attacking oppurtunities. As great a player as he is, he doesn't have the skillset to carry an entire side on his own, something Hayne has proven to do.
The question is, if they swapped sides, who'd perform better and fit in easier? Hayne at fullback for the Storm around quality all over, or Slater in the Eels who have strong backs, forwards, but lack direction at half and hooker? That's the question that should be asked to determine who is the better player. Obviously Slater has been more consistent over the past few years, but so have Melbourne (illegally) and QLD which makes it a lot easier for him to be consistent. Put him in an overall weak side, would he be as consistent? I highly doubt it. Players rarely stand out in poor performing sides. It's a 'what if' but it's important to keep that in mind.