Why are they necessarily exclusive? If you admit four free states from Canada, it just means you have to admit more slave states to balance them out. And the New York Herald knows where to find them:
'the most superficial view of the annexation of Honduras will convince the intelligent reader that it is a question of more immediate and comprehensive importance than the Cuba question... the State lies in that exact geographical position which will enable us not only effectively to check the machinations of British, French, and Spanish emissaries, and to upset their schemes for the 'balance of power' upon this continent, but the suggested annexation will give us the nucleus around which we shall be able most rapidly to bring in, upon the same terms, the other States of Central America, to the west and the south, and of Mexico on the North." (New York Herald, 7 June 1854)
'the prevailing conviction of the American people is that sooner or later Cuba must be ours... their prevailing desire is the earliest practical movement of the government in this direction. The Kansas slavery agitation of the last four years has left the democratic and the opposition parties of the country in an unprecedented condition of disorder and disintegration. Upon the remaining fragments of their late slavery and anti-slavery party capital, we can expect nothing in 1860 but a loose, disorderly and riotous scrub race for the next Presidency, and a contest in the House of Representatives which may abruptly break up the general government and the Union. But with the general rally of the scattered elements of the old democracy upon this auspicious party movement for the acquisition of Cuba, all the existing lines between sections and factions will disappear, and the democracy, from the brink of destruction, like Napoleon in the critical moment at Marengo, will be enabled to charge back upon the enemy and win the field.' (New York Herald, 18 January 1859)
Bear this kind of thing in mind the next time people tell you 1860s Britain is an expansionist power.
don't get too excited about the New York Herald (or really any newspaper for most of the history of the US). The owner (Bennet) of that paper said that the function of a newspaper "is not to instruct but to startle and amuse."
of course the United States was an expansionist power.. Manifest Destiny was little different from European powers doing their own imperialism. The US got to keep its empire though.