I tried both on our last trip, and find it hard to compare the 2....I love Saana, the menu is great and so is the atmosphere, it's loud, more casual, and has inventive choices on the menu. On my trip in June I stayed at aKV by myself for 4 days and ate there 3 times...
HOWEVER - we tried Jiko on our last trip too, and it was above and beyond a GREAT experience for us! Our server Nasser was amazing, the food was stellar, and the entire experience was phenomenal. I had the BBQ chicken flatbread, which was one of my favorite apps on the entire trip, and the filet with mac&cheese, and it was delicious - the filet was cooked to perfection and the red wine sauce complemented it perfectly. The Warm Malva Pudding was also great...
If I had to choose one, I'd choose Jiko for sure, although that being said it doesn't take away from the fact that Sanaa is amazing too!