38% is 19% extra damage on average from mastery, with increased RNG because bigger swing, no effect on pet.
12,8% crit = 12,8% extra damage on average without crit damage talents/modifiers, with reduced RNG from crits. However we get double shard chance on a immolate crit tick and that means more CB's and less incinerates. However I have no idea how to calculate that in a damage increase.
Both effects are multiplicative with eachother, so I could very well see both being desired in equal numbers. Besides that I find those ratings to be pretty close, especially when you use a pet.