On The Fly - looks better to me and it seems to be a lesser used way to build. It should make a fresh new approach to base building.
Both the videos were far too short, but we got the point in the more well used panel / component creation before - placement. Its ARK like, but even 'The Forest' (game) did it better than ARK, (just my own opinion).
As usual, it's not black and white. I like the "before" mode, largely because I'm more familiar with it from other games. But in the (too short) demo, more complex objects were being created in the "on the fly" mode. I voted for "before" but I would like to see more demonstrated in that mode.