Is both an option? Where are you staying? How long are you going for?
If you are staying at AKL, I would make it a point to eat at both (and don't forget Boma, too). We just returned from an AKL stay and had a great Sanaa lunch, Jiko dinner (part of the Wanyama) and Boma dinner and breakfasts.
if I had to pick dinner at Jiko or Sanaa for lunch, it would be a tough choice, but I'd probably choose Jiko for a nice dinner. Sanaa is everything you read and more, but taking time out of your late morning/early afternoon, especially if using Disney transportation, would not be the best usage of time, if time is a concern, IMO. You can still explore the AKL grounds and see animals on the savannahs at night, but you can't guarantee getting a window seat or animals being close to your window if you get one at Sanaa.