I tried both (Sakura and AC) during my stopover back in March. Both are very, very nice and spacious. Sakura Lounge was the bigger eye-opener of the two as it's much better than before the refurbishment last year. It helps that OW elites and premium passengers can access the two lounges if that's what they want.

Overall I give an edge to the Sakura lounge because of their
A) More extensive food options (soup, rice dish, salad bar, munchies)
B) Full-service bar in the afternoon and evenings
C) Free massage service (must try!)
D) Nooks and crannies in which to relax