Hello Gonzalo, and welcome to the friendly Camera Labs forum!
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You have a Nikon 28-300 VR plus a 18-300 VR? That's quite some overlap.
I have both the Nikon 28-300 VR
plus the Nikon 70-200 VRii
and I must say: for a Safari the 70-200 simply is too short - although the image quality is better than from the 28-300 and the 18-300.
On the other hand the Nikon 200-400/4.0 VR is a monster of a lens in size/weight and price. Unfortunately the Nikon 80-400 VR
is quite outdated although image quality is not bad.
But perhaps you should consider the Sigma 120-400 OS
or the Sigma 150-500 OS
(I've linked to my reviews).
Thomas (beware: Nikon-fanboy and moderator!) My Lens Reviews
, My Pictures
, My Photography Blog