I've been told both the Engel and Waeco fridges share the same compressor: Danfoss BD35F The outside of the Engel is supposed to be a little more resistant to wear (stainless steel outside), but if you make some sort of (wooden) box around the Waeco, it should be just as good (take care not to block the airvents). Waeco also reaches -22 C inside under Southern Africa conditions.
In Holland there's a price difference between the two (40l versions) of €225.
I think the weigth is of less importance when travelling with an exp.vehicle (never overload of course!), but an extra 10 kg won't do any harm.
So, I would go for the Waeco Coolmatic!