us and united-states
I think the later is better; in any case, both are synonymous.
The merge has been completed, and the two tags have been set up as synonyms.
In response to the comment, as a moderator I'm fine with people suggesting that we manually efect tag synonyms & merges. On a site like Stack Overflow with hundreds of thousands of visitors, the likelihood that enough people who meet criteria (>5 upvotes on tag) will view and vote on tag synonyms is pretty high; on a small site like this it could take forever. Also, we have tools to automatically remap one tag to the other, reducing required work.
That said, you can go here to see outstanding merge suggestions and vote on ones you like. For each tag pair, click on the leftmost tag as shown below:
From there, you can vote:
With that knowledge, in the spirit of mid-1950's organized crime, "vote early, vote often!"