You don't give much criteria for the choice. Are you looking for an upscale restaurant? A quiet place for 4 adults? Is it a joint "date night" dinner? Do you want more casual service? A more relaxed atmosphere? If I were to choose one in a heads up comparison, then Jiko wins simply because it's a bit more elegant, quieter, and a more unique experience (unless one dines early enough to catch animals Sanaa). The food at Jiko is exceptional, though you pay for that as it's a Signature, and the wine list is topnotch. That being said, Sanaa is an excellent eatery in its own right. The bread service alone is worth a trip to Sanaa. The service is good, the food tasty, and the wine list also quite solid. If you dine before it gets dark, then you do get the benefit of likely being able to see animals outside the windows. Certainly, the cost is significantly less at Sanaa which may be an important consideration. You will be fine with either choice, but choose Jiko for a special night and Sanaa for a more convivial, casual outing.