Thanks ! This is an Ultimaker gantry with a different Z axis.
On a Prusa you have to move both the heat bed and the hotend. On this one you just have the hotend that moves, less weight, less inertia, better acceleration. The bigger it gets the more weight you have to move on the Prusa while it is less in the case on this design.
Dual Z leadscrew is a bit better than the single Z to avoid the heatbed to move, it is the same idea that you have on the Prusa for the Z axis, because if there is no leadscrew on the other side there is always some slack and the longer it gets it is way worth: introduce source of vibration, inconsistent layers.
If I have to upgrade/rebuild one, I will probably put MGN12 for all linear guides, it starts to be cheaper from a price point of view.