I love them both for a number of reasons. But most of all, I prefer Kidani.
I do not think that Jambo is more lively, I just think the lobby area is more busy. Two different things if you ask me.

We love the pool area at Kidani more than Jambo....much more activity and the pool parties are awesome! Kidani does have a gift shop...much smaller than Jambo but you can use your refillable mugs there which is nice and they have the DVD rentals. Also, Kidani has an awesome community hall. Great crafts, pool table, video games and movies. We spend hours there! And the view of the savannah from there is amazing!!!! The kids love playing the games and doing crafts!
The walk to Jambo is nice, or you can take the shuttle. Both very easy. We do love Mara at Jambo but unless you are eating there each meal, it isn't a big deal to us to be at Kidani and walk over.
The staff at Kidani are very nice, Sanaa has great food, and the savannah areas are wonderful.
I LOVE Jambo and we take the kids over there for certain activities, like free cookie decorating in BOMA and the medallion rubbing, but we love the rooms and atmosphere of Kidani.
You can't go wrong with either!

Hope this helps!