Gives a significant boost to small countries. The US would benefit greatly, of course, from having about fifty times its current surface area, but Israel will have nearly 20,000 times its current land. Countries with relatively poor conditions like Mongolia are obviously going to disproportionately benefit as well.
There's a fair chance most of these new Earths are going to rebel in pretty short order. Generally speaking, the people who are willing to go out and carve a new life in a completely virgin land, absent even the slightest of amenities and modern conveniences, are not the kind of people to meekly put up with any kind of guff from a distant ruler. Further, they only need to hold a 6-meter chokepoint, a rather easy win condition even if they're relatively poorly equipped for war.
That being the case, I imagine a number of more oppressive regimes will try to put massive limits on who's allowed in in the first place... which will hamper development significantly. Other nations, particularly those who have pretty much only trash land in the first place, might take everything through and move their capital over, largely abandoning their Earth Prime holdings.
Over time I expect most nations to do this, actually. There's a lot more stuff on the other earth than any single nation can get here. It's just a matter of building enough population and constructing enough infrastructure to use it. Eventually Earth Prime will become the world's largest open-air trading hub as few nations use it for its land value anymore, just to exchange goods with other alternate earths. Europe will be significantly advantaged in trade over other nations, due to having a large number of small countries bunched up with pre-existing infrastructure to move goods between them. The US will have a modest advantage due to the Mississippi Basin giving it good internal transport, but there's only going to be a handful of portals to trade with in the first place, mostly shipping from the Gulf of Mexico and South America.
There's no indicator of what happens when a country is conquered, splits in a civil war, etc. Assuming the portals are fixed and immovable, expect a certain amount of conquest followed by the alternate earths putting together the most comprehensive tower defense games around their portal they can manage. If a new portal forms when a country splits, I'd expect most nations to rapidly fragment themselves along existing boundaries in order to generate more portals and thus more planets to colonize.
