Transforming into an ugly pos isn't Sasuke's style, imo. I like Susano'o better as a "transformation." Likewise, I'm glad Naruto doesn't turn into an ugly, sinister-looking beast anymore; it's better for him to look "Sage-like" and emit bright chakra.
Basically, they both fit their own themes better in the War Arc; Sasuke shouldn't gain "bodily" power-ups, since he's of he Yin/Spiritual variety, whereas Naruto doesn't need to transform into something "dark" or sinister, since he's of the Yang(light)/Phyiscal variety.
Kishi also made Senjutsu more of Naruto's thing; since the Curse Mark is Senjutsu, it usurps too much of Naruto's style. It'd be like if Naruto gained the Rinnegan.