+1 for Öhlins
I would definitely go for the Öhlins... ok I already did
Have it now for nearly a year in my Z. Did perfect in Iceland, even with 2 heavy guys on it (when the BMW from the buddy broke down).
1. the difference to the original shock is big. You think you drive a different bike. The handling is more precise than before and the damping way better (and adjustable).
2. I think the reliability is better with the Öhlins. I read from a german guy (Jan) whom failed the Wilbers on his Z twice on his way to South Africa.
Although you have a (small) external reservoir which Wilbers has not. Helps reducing the temperature when fully loaded.
3. Most shocks are to big for the small space in the Z. Therefore with many shocks like Wilbers the rear wheel travel is reduced (by 1 or 2cm). I know only about the Öhlins which nearly has the original travel (by 1-2mm). The shock from the TT mentioned above will not fit in the Tenrere.
Thats why I took the Öhlins and espicially when you are doing such a long trip I would recommend it.
If you have any questions about it, feel free to ask.