ok here is my hypothetical test:
each car has just the driver in them, the car is stationary and is hit from behind at 30mph.
both drivers will receive whiplash so equal on a rear impact.
the car is hit from the passenger side at 30mph, both drivers hit their head on the window.
again equal on injury for passenger side impact.
the car is hit from the drivers side at 30mph. the micra's B piller collapses and the side impact bar does little, where as the volvos shifting seatrail idea moves the driver further away from the impact. the volvo driver comes off better, but with all side impacts there is little between them.
the car is hit from the front. the volvo has a longer bonnet, but if hit directly in the centre (maybe by a motorbike) the impact will push the engine into the cabin area, the engine has about 400mm of stuff between it and the outside world. the micra has more space between the front of the engine and the outside world, but also the engine is sideways, increasing the surface area, which makes it harder for the engine to be pushed into the cabin area.
so for a low speed direct centre impact i would imagine the micra coming off better.
the car rolls into a ditch. the volvo is 2 tons, the micra 1 ton, when upside down there will be more weight to compress the roof in the volvo, but also more surface area to support it. so i would put that as equal.
there is snow on the road and the car is in a low gear. the volvo with its 2.3ltr engine, close ratio gearbox and rear wheel drive, has no trouble breaking traction. the micra with its small 1ltr engine, front wheel drive and fairly tall gears will grip the road a lot better in the snow. i would put that as a win for the micra.
the driver falls asleep at the wheel, or has a minor impact that knocks them unconscious, and they press down on the throttle. the micra is front wheel drive, the first front impact will damage the front wheels and the car will stop. the volvo is rear wheel drive, and even if the front wheels become damaged, its powerful engine and rear wheel drive will cause it to "ricochet" off walls and other cars etc, causing multiple crashes (this is information provided by skid pan instructors on why front wheel drive cars get better safety ratings). i would put that as a win for the micra too.
so in my opinion they are pretty much the same but the micra comes off slightly better, plus to drive it feels more stable and responsive. at 70mph the volvo feels like a boat, wobbling in the road and has a much larger braking distance, it is very noticeable how much extra weight is in the volvo when trying to stop it, and it regularly breaks traction in the snow.