Sorry for late replies, I've been out of town.
Sault Ste. Marie would be larger, and have more development. The Great Lakes in general would be better developed and more populated.
Sault Ste Marie I think could definitely be larger, for the reasons you suggest. Winnipeg would most likely be smaller, because IOTL it benefited greatly from being on the only major land route between east and west Canada and ITTL that route isn't going to exist, because as previously mentioned with no border there's no reason to build a railroad across the Canadian shield when traffic from Montreal or Toronto to Calgary or Vancouver can just go south through Chicago instead.
Thank you! This will certainly make the Great Lakes region more interesting, with a large number of Canadian cities accompanying it.
Also,
@Thisisnotausername, though I agree that a Canadian Shield railroad is likely improbable, could you possibly foresee a railroad or railroads being built that North in order to speculate/sell off the land similar to the American West? A route could be built through the Canadian Shield to reach the Canadian Prairies, which could possibly be lucrative for a railroad company to sell off, eh?
OTL Canadian Maritimes would be more industrialized, remaining a part of New England. Boston would probably be far less Irish and Catholic, with most of the migration going to Quebec and Montreal. Perhaps PEI would be recaptured by the British to salvage their Naval presence in America after the loss of Halifax in the ARW, especially considering PEI and New Brunswick were basically completely unpopulated at the time.
I can see New Brunswick becoming a state like Maine did to boost the number of anti-slavery Senators during the Civil War. I also think that the states might look pretty much the same in most areas, considering that 1. they are pretty good shapes now, for the most part, and 2. the federal government had way less power back then, and couldn't dictate that much to settlers, even when it wanted to. So unless the settlement patterns of this US are changed, which I don't think will happen only with the US having Canada, then the states can be largely the same.
Also Chicago would probably be bigger, as would Seattle.
Montreal could possibly contend with New York via population metrics. The states of "Greater" New England would probably collaborate with each similar to how they did in real life. Prince Edward Island could have a larger population, considering it has potential for massive diversification of its economy, especially for its region and because its restrictive land owning practices would never come to pass due to being under the United States.
I also thought that New Brunswick becoming a state alongside with Maine would be beneficial, as it allows Abolitionists to simultaneously bolster the anti-slavery effort while appealing to the Canadiens, thus continuing the mutual
tolerance that each group had for each other.
Lastly, I was doing some digging through old AH threads on mobile, and found various threads referring to
GRAND Canal, and the NAWAPA project, both proposed projects for providing water to the Arid West. As much as the plan is retrospectively stupid and destructive, perhaps this United States is more interested in doing so. Just providing more food for discussion.