Walls and Ceilings - Cutting Drywall/plaster to meet a seem of greenboard

In my bathroom where I am installing a new Tub/shower, the alcove is getting new wall straight to the ceiling everywhere except on the far tub wall. That side will have to meet up with the existing material which is Plaster over drywall. I need to cut the plaster over drywall nice and straigt from floor to ceiling and meet up with the greenboard coming from the tub alcove. What is the best way to cut the plaster over drywall? I own both a dremel and this thing

http://www.sears.com/sr/javasr/product.do?cat=Portable+Power+Tools&pid=00917246000&vertical=TOOL&subcat=Rotary+%26+Spiracutter+Tools&BV_UseBVCookie=Yes

ANY tool with the abrasive style blade or roto bit will work. Or turn a dull saw blade, you never use, backwards in your saw. That way you can control the depth, by using a saw. (This will create blinding dust, BTW, without heavy ventilation/exhaust.)

If you predrill a hole into this seam location and were to luck out and there was solid wood under it or even close by, you can effectively just use a chisel to cut, without getting all that fracturing from the wall springing. This method obviously creates the least amount of dust in the air.

I suppose one could also use one of those abrasive grout removing hand tools to at least get a straight line cut into the plaster before cutting through with a chisel or wonderbar, if you are a perfectionist and want a neater looking break at the edge of the plaster.