Finished Product
WIP
Since the first head isn't entirely yours I won't comment on that (you should write it in the thread tbh), so I'll give you some suggestions for the robot head.
Now im, not an artist but that second head doesn't look right the shapes on the top(and around the face) don't even look like it's finalized it looks like a sketch. The teeth look stolen and enlarged from something I've seen before and it looks pixilated. 3/10. But keep making art man!
One way to improve and learn new things is to just trace stuff, and i mean fully redraw, for instance: toribash head textures or some random illustration of the net. (of course that would only be for practicing so you shouldn't do anything with it afterwards besides sharing your progress).
I Think on the Robot head You Should've made The lines Thicker For the Markings, The Lines being Thin Kinda Works, but You should've made it thicker.Also The Eyes Look Really bad You should've Made them Look more Solid Rather than using an Airbrush. i know it's Just An WIP but..