I don't think Messi or Ronaldo will have that great of a WC - Portugal simply don't have the quality to go far in this tournament this time round and Ronaldo IMHO, is playing with quite a limited squad - and on top of that has been carrying two injuries. Messi has produced moments of individual brilliance, but has lacked an overall good performance. He too, is playing in what looks to be a dysfunctional team which looks wobbly defensively - unless Argentina's overall team play improves, I don't see Messi moments being enough in itself to win them the WC.
Personally, I think Messi playing well, irregardless of whether Argentina win the WC or not will be enough to cement his legacy. One player can get you far in this tournament, they can even get you to the finals - but it's extremely rare, if ever that a single player wins you the WC, so it is unfair to expect Messi to do everything when football is fundamentally a team sport.
I think the players who will do well in the WC and have a chance at the golden boot are Robben, Muller, Neymar, and perhaps Klose and RVP. Robben in particular has been the best player in this WC so far for me.