From my experience, it can be a little harder to rank .us outside the United States, but it also doesn't make a tremendous difference otherwise.

The TLD doesn't matter, what really matters is good optimization. I have domains from some of the most exotic TLD's (.dj, .ms, .vc) and they all do just fine once properly optimized.

Sorry p2y, I totally disagree, the most relevant TLD to the country will win hands down if both are optimized perfectly.