For the longest time Jiko was almost universally considered the best restaurant on property after V&A. But apparently recently the head chef moved to California Grill, and as a result Jiko's reputation has fallen a little, while CA has taken its place as best non-V&A restaurant.
I've been to both. I love the theming and details of Tiffins more, although the atmosphere in Jiko is great and there's a couple nice little hidden details there as well. There's more of a backstory with Tiffins and Joe Rohde was more involved. Love the music in Tiffins too (can't remember if Jiko has any music playing in it).
As for the food... I gotta give it to Jiko, just slightly. Now I went before the change of chefs so it definitely might not be as good anymore, but Jiko was one of the best meals of my life. I had the lamb chops, which were absolute perfection. I tried the steak as well and it was out of this world. But Tiffins was also fantastic. I had both the pork belly ramen and duck at Tiffins and they were both excellent.
Both were two of the best meals I've ever had at Disney World. Right now with the chef change at Jiko I might pick Tiffins over it. (Tiffins is a little cheaper also, I think.) But you'll have a great meal either way.