personally for me it would depend on the horse/pony in question.
main question would be could they live out happily and not suffer/require massive amounts of food to keep warm and not loose weight?
i am not keen to have a horse on its own at all but i also wouldnt be able to keep my TB out all winter even with companion with no shelter even if rugged up to eye balls he just wouldnt cope.
Also how much natural shelter is in the field if they ahve to stay out?
I guess every situation is different but many will obviously say with companion as horses shouldnt be alone but i would prefer a happy healthy horse on its own than a miserable shivering wreck with a companion
As long as there was natural shelter and the pony did not have any particular issues which means it couldn't cope I'd go for living out in company, even there was not a field shelter.
ive got masses of natural shelter, but nothing man made. Mine are happy in there living in their little herd (even the tb, who no longer thinks she should be made to stand in a stable lol).
If its a big bald open field with the wind and rain belting across it, not so good...
I would definitely go for the company. Is there natural shelter? If not then as long as they are rugged up well with ad-lib hay/haylage they'll be fine and happier with the company in my opinion.