All the talk of Hillary Clinton winning in a landslide against a good GOP candidate is pretty silly. Clinton's approval ratings are so high because she, until the Benghazi story broke, was out of the political spotlight for 4 years.
From the time that Barack Obama won the Democratic nomination, until the Benghazi attacks, the Republicans saw no need to attack Mrs. Clinton personally, because they had President Obama to attack. Now, Hillary Clinton represents a real threat to their chances in 2016, and therefore, the Benghazi story gave the GOP a chance to renew their attacks on her. The result is that, in certain polls, her approval ratings have dropped from the low-mid 60s to the low 50s. This pattern has been repeated throughout her history in the public eye. When she is involved in something political, her approval ratings drop - when she is seen as apolitical, or at least outside of the fray, her approval ratings rise. If she runs, expect her ratings to drop fairly quickly.
Regardless, Mrs. Clinton is an undeniably strong candidate, in large part due to her experience. She could come close to clearing the field of any challengers to the nomination (something no other candidate could do). However, I'd be surprised if these advantages amount to more than her running 2 or 3 points better than a generic Democrat candidate.
So if Clinton runs 3 points better than a generic Republican in a normalized election, you've got this map:
However, Christie is potentially a very strong challenger - he's an incredibly popular Republican governor in a safe Democrat state, is seen as bi-partisan and pragmatic, and embraces a "no-nonesense bluntness" style of getting things done. He may have trouble in the early primary states, but if he manages to win the nomination without veering too far right, he could also run 2-3 points better than a generic Republican.
So if Christie runs 3 points better than a generic Republican in a normalized election, you've got this map:
It will all come down to which candidate runs better than their generic counterpart.