Johns. He led Newcastle to the holy grail. Twice. Thurston has had some very ordinary years with the Cowboys. And his sole premiership was as a late replacement in an interchange role.
Johns is a significantly better defender than Thurston. There's times the bloke is not much better than a turnstyle. I think people overrate the 'gutsy' side of his defence because they know he's generally not expected to make a decisive tackle, and when he does, there's much fapping. They also talk of his commitment in defence, often his scrambling tackles are off the back of an initial miss by him.
I'd say Johns had a more creative kicking game, and he made a lot of opportunities from something instinctive and most times he went to that type of trick play, it was executed brilliantly.
I think rugby league has this real 'recency' bias to it. In the last era Lockyer at one was rated by some as the greatest ever. Then Johns. Buderus the greatest hooker of all time. Thurston here is the best of the bunch in this era so typically the media faps the same praise as they did on the previous eras holy grail of playmakers. I find it cringeworthy often that guys like Daley, Sterling, Meninga are often quickly forgotten and the flavour of the day is touted as the best of all time. Obviously, there were more greats before that era as well.