I don't trust head to head polling after what happened 4 years ago. In almost every head to head poll, Clinton beat Trump.
Also, you have to be able to win in the electoral college, so polls in swing states are probably more relevant information that national polls. But even saying that, it's still too far from the election and the attacks won't really begin in earnest until it's clear who the Democratic nominee will be.
(I'm not saying this as an anti-Sanders argument, btw, because I'm actually warming to him, but just to point out that the polls can't be trusted. Heck, the early polls showing Biden with a comfortable lead in the national polls may have been true at the time, but that support seems to be vanishing quickly.)
I do think that Sanders could win, but not because of any national head to head polls. Those failed to predict last time.