We've eaten at both...and Jiko is wonderful, but Sanaa is among our very favorite restaurants anywhere...
Jiko is much more "high-end"...(also, not surprisingly, much more expensive)
Sanaa is more ethnic, but we found the food beyond wonderful...they have a lamb patty--kind of like a hamburger--on a piece of crispy pita-like bread with the most marvelous barbeque-y type sauce, that is to die for. The curries are wonderful, too, as are the chutneys. And the desserts are SO worth the calories!
LOVE LOVE LOVE Sanaa!
Jiko has steak, lamb, fish, etc ... it's also got some ethnic twists, but it's more like a high-end "regular restaurant"...really really like it...
Love Love Love Sanaa...really really like Jiko.
Boma is also very good, for a buffet restaurant. And Mara is one of the better food courts.
Can you eat at both Jiko and Sanaa? They are very, very different experiences (and w/Jiko at Jambo and Sanaa at Kidani, you'll feel like you're in different places, too)...
Good luck...