I have a 1932 English cottage. The lathe and plaster were beyond salvage. New electrical is going in now. The goal is smooth finishes walls in the living room, dinning room and kitchen areas. Smooth ceilings elsewhere and sand finish on the walls in the halls and bedrooms. My exterior stucco guy is recommending imperial plaster for a more authentic look. What are people's thoughts and experience? Is it worth a 15% premium in price? How are repairs and durability? Is one more eco friendly than another?

I'm not sure what "imperial" plaster is, (brand name?) but in my book plaster is definitely a better finish than drywall. My experience is that nail pops and visible seams, common on drywall (especially aged drywall) are rare with plaster. It is a harder finish and flatter; with drywall you can see the "paper" surface; plaster you can't. It is more of a mess going on since it is essentially a liquid, but well worth it.
I've done many projects, either remodeling in antique homes or reproduction antique homes, where an old look was essential. In these we use two coats of Structolite, with the 2nd coat troweled as smooth as possible. Structolite is the base (brown) coat for the old fashioned and rarely used two coat system (brown coat, lime coat). It is somewhat sandy and when well done is almost impossible to differentiate from antique plaster. Obviously if you try this, do it first in a closet or somewhere where you can see it before doing room walls.
One word of caution: plaster on top of 1/2" blueboard will be thicker than just 1/2" drywall, so door and window casings may need extension jambs.
Bob

You mean it's just 15% more for lath and finished plaster as opposed to finished sheetrock? Just opinion here because I don't know much about it, but that sounds like a good deal if your guy is good at it. I would think it would complement the period look of your home.

Just curious, what was the old lath like? Were there thin horizontal wooden strips or just metal?

Jelly;

in 1932 probably wood lath strips and plaster very often was reinforced with horse hair, later on gypsum lath was used

I guess I am officially a construction geek when I get excited about 30's era wooden lath!

Round these parts Spanish Moss was often used like horsehair, and going way back it was mixed with mud and used like adobe.