Its a century old home, its uninsulated, i am wondering in summer and winter am i better off with it or drywall from a heating and cooling perspective (energy cost)

Also if i insulate the walls and attic does this change anything (i know it will reduce energy use, i mean the thermal mass helping or reducing energy usage) as i plan on doing this this year and next.

If the plaster is in good condition, and you can insulate from the outside, the work will be much less disruptive and probably less expensive. As to mass, probably depends on the thickness of the plaster. Since it goes on as a "liquid"$, plaster can be a better air seal.

We work on a lot of homes on the National Registry and find none of them will pass a blower door before we install insulation. Thermal mass is not a significant factor in stick-built homes over basements, save the basements themselves of course.

Plaster can be a better air seal than drywall, but most air leakage is not through hard surfaces anyway, but through joints and intersections of trim and other surfaces, or through obvious "holes" in the walls like outlets and lights.

Agreed.

Foam the rim joist and call me in the morning...