Arkansas is definitely more likely: Arizona hasn't swung any direction since Gore. Well, okay, it has, but not in the direction Democrats want it to (Bush won by 6 in 2000, but since then its been 10+ margins). It seems pretty inelastic at this point, but I assume Hillary could get within 5 of it, maybe even 3 or 2 if its a particularly bad GOP nominee.
Barely 10 in 2004
8.5 in 2008 despite having a Favorite Son R
9 in 2012
Recent polls show Arizona going very close. Demographics (fast-growing Mexican-American electorate) suggest that Arizona is going in the same direction as neighboring Nevada and New Mexico and one-point-neighbor Colorado. Slower, but inexorably.