We have stayed at both. As a resort overall, we feel Jambo is better with it's three restaurants, larger lobby, larger store, and larger pool with lots of areas to find shade. However, we will stay in Jambo only when using studio or GV. The studios at Jambo and Kidani are about the same size, the GVs at Jambo are the best at WDW. But if getting 1BR or 2BR, Kidani wins by a very large margin, the room is 100 sq ft larger, you have an extra bathroom, more closet space, separate tub and shower in master bath (Jambo is a combined tub and shower with a very high stepover into tub), and just all around nicer rooms. Kidani does have the better kid's play area at the pool, one of the best restaurants at WDW (Sanaa), and ideal parking under the building if you have a car.
And the walking distance front door to front door between the two is about 1/2 mile.