blend horsehair plaster and new patch of... ?

[mrbigstuff]

I'm hesitant to try my hand at plaster, but I'm ok with drywall and taping. But, my wall seems to be part plaster and part drywall (have yet to take it down). I need to blend the two, and would prefer not to demo the whole wall but would consider it if it's a crappy idea to blend these two systems.

What would you use to patch the gap between the plaster and the drywall?