I don't know what your plan is with this area but why not seed the slope and cover it with erosion control blanket? Use a temporary straw blanket with staples. Plastic sheeting will kill any vegetative matter which may try to grow and it is the roots of these plants that will hold your slope best.

I agree with "more to grow."
If that's too costly, try building swales, and planting them with fast-growing vegetation, and more longer-lasting slower-growing vegetation.
Additionally, plastic won't keep the land from eroding. Rain has an inevitable way of getting under plastic and eroding from underneath. Plus, you will kill whatever native seeds you may already have in the earth.

Here is a link that might be useful: Desert Gardens