I am not sure what u intend to shoot up there... but if u are thinking of panos and joining multi shots then take the 16-80 mm... no harm taking a few more shots... I have joined 23 shots in one pano before, so it's not much of a problem... I recommend the 16-80mm as the distortion is more controlled... this means that it is easier to stitch your pano shots together... more usable part of the image is available when u crop...
But if u are thinking of taking wide angle one shot type of photos then the 11-18 is unbeatable where that is concerned... u get true wide angle at 17mm after factoring in the crop factor of 1.5X.
Since going to Mt Kilimanjaro is once in a lifetime (pretty ex huh?), then bring more lenses... use a hipster pouch (from Lowe Pro) to holster the camera plus one or two light lenses... keep the heavy lenses in the back pack...
I would bring:
11-18mm (very light)
100-400mm (or a long prime 500mm)
These are inexpensive lens, but definitely allow u to cover anything and everything... which is the objective here right? Unless u are like Michael Reichmann... can go on expedition and carry a helicopter load of stuff... don't forget u still need to carry flashes, batteries, chargers, memory cards, backup HDD, laptop, cleaning gear, tripod... siong!