I grew up with vermiculite (now rare due to most of it being contaminated (in the ground) with asbestos), moved on to fiberglass, and am now a blown-cellulose convert.
They all work, for various values of "work."
Cellulose is inexpensive, gets MORE effective when cold, and blocks airflow well enough that a vapor barrier is (possibly) optional according to some researchers. It's dusty during install, but otherwise innocuous. I'd suggest getting a bale from each supplier (or each different brand from suppliers) and inspecting it - or from the supplier you'd go with on a cost basis (first) and if that fails your inspection, from others. I found the sample I got from a major home improvement chain was contaminated with a lot of scrap plastic; as it turns out, not only were the bales from my local home improvement non-chain cheaper per pound, they were also good clean cellulose with no plastic scraps.
Fiberglass compares well at 70F (where R values are measured) but gets quite terrible at -20F, just when you want it working hardest. It's far more prone to air movement (whether in batts, where seams are are a problem or as loose-fill.) Plus there's that whole scratchy thing going on with glass fibers.
According to "belt and suspenders" thinking and "it's cheap enough" my cellulose is sitting on top of a vapor barrier. One more way to stop air movement. Since your climate is primarily heating, your vapor barrier goes on the inside (generally, the "warm" side - complicated in climates where heating and cooling are similar.)
If you are not using the space beyond the wall, insulate the wall (and perhaps add some furring strips to get more insulation on the wall where there is room.) If you insulate the roof, you need to provide cold air channels against the roof surface to vent the roof appropriately (though with that steep of an edge, it might be difficult to get a serious ice dam, which is what roof venting/cold roof design is trying to prevent.)
Since you are gutting it, you may also want to furr out the endwall to make it thicker and give you more space for insulation, since you won't get a lot of R-value with any insulation in the space available. Alternately, and at higher cost, you could sandwich a layer of sheet-foam type insulation over the studs and under the drywall on that wall.
Generally you will also want at least a few cans of polyurethane spray foam. While you can do all your insulating with spray-foam, it's very expensive, relative to other kinds of insulation. But it's great for sealing irregular cracks and crevices.
Best Answer
If the house is old enough to have a plaster ceiling, there is a terrific chance there is no insulation.
You would be wise to cut an access somewhere and get rent a blowing machine for insulation. get about 16" up there. Taper it to the roof rafters so it does not touch the roof. It will allow the roof to get too hot in those places and deteriorate the roof prematurely. 2' of airspace over the insulation is recommended.
Be mindful of any added can (recessed) lights that were put in later, they need to be IC (insulation contact) rated or keep the insulation off them too.