Manga-wise, I'd say OP. It has a high amount of intricate world-building compared to FMA, which only expanded on Amestris and Briggs, with only a little info being given to Xing and none to anywhere else. Character-wise, I'd still say OP as it has at least equal quality, yet superior quantity when it comes to developing the cast very well. Whilst FMA's story probably has somewhat superior writing quality, OP has far more directions that the story could take due to the numerous factions within it, whereas FMA was an open and shut case once Father was defeated.
Anime-wise though, it's more difficult to say. Whilst I'd be inclined to say OP due to how part 1 was done, it's started to show the typical Toei problems such as poor animation and slow pacing in the recent episodes. Even with part 1, there was just as much bad filler as there was good, such as Luffy using a 1,000,000 degrees C attack, which you'll find in Marineford, is complete BS. Overall, I'd still be inclined to say I'd rewatch select arcs of OP over marathoning FMA:B again, primarily due to far more emotive scenes and a superior score, though as far as being faithful to the original work, FMA:B takes the vote by a mile.