Thoughts:

I actually like these pieces the most, the first one in particular. I think the first has a good contrast between the two objects in the piece, the branches and the car. The car is much darker and its shape is much more free form than then the branches, which are tonally lighter and similar and have sharp edges. I think it is easy for the viewer to clearly separate the two types of lines, and group them accordingly into two different objects because of the differences.  Compositionally I think it is balanced because the weight of the dark car in the bottom right is balanced by the quantity and size of branches in the top left.

The second image doesn't have to deal with grouping separate objects as much because the shapes are bigger and more clearly connected.  Because of this I tried to play a little bit by making the branches and car a very similar color, with the attributes they have (leaves and wheels respectively) more defining characteristics of the object. Overall I feel like between the clear geometric shape of the wheels and the more free form leaves it is easy to  distinguish the car as man made and the tree a natural occurrence.