PharoahIII:
As a writer, I think I am in a better position to comment on this topic. Firstly you must appreciate that Achebe's mastery of the English language was second to none, although he did not have a flair for words as Soyinka does. In terms of scene depiction, Achebe was a genius. Achebe could describe a scene like none other. Achebe had total control of his audience, he knew how to keep them on their toes, wanting and practically begging for more. This is a feat Soyinka simply could not achieve. But in terms of consistency, I'll give it to Soyinka.
Soyinka's best work till date remains Death and the King's Horseman (The first time I opened this book I knew I was in literary heaven). If one were to take this work away from Soyinka, Soyinka will still be Soyinka. But if you take Things Fall Apart away from Achebe, Achebe will be a nobody. He'll be like a disrobed emperor. His other works were quite quotidian. Achebe was a master storyteller and writer, and Soyinka, more of a writer than a story teller.
If I had to choose between the two, I'd choose Soyinka, and that's only because storytelling in itself is a more difficult craft to master than writing.
Things fall apart is not a book, it is a bible.