both, but it depends which one fits the use better. Big business won't wan't .biz. They want .com. However, I see .biz being a very good fit for small businesses where there are hundreds of people who would want the name. For example, I have tried for names like exterminating.biz, wrecker.biz, or jobplacement.biz. These are generic domains that have potentially many thousands of potential end users of the domain. Something like Rocketfuel.biz might be okay, but the potential number of end users is small and they would probably already have .com names anyway. I also think brandable names would find a way to make .com work with a longer name, so I limit .biz to generic names. Also, if it isn't a business, it doesn't fit and I wouldn't register a biz.
For .us, it is broader in that it isn't limited to business types of use. However, it is more geographically limited. Not only the connotation of being a US business, but the Nexus requirement could limit the market for development. If you WANT to designate a business as US based or a US branch, it's superior for that purpose. I have a lot of geographical domains in .us because they make sense. Names like OhioRiver.us, MammothCave.us, HarrisonCounty.us, and ManhattanNewYork.us.
I like both, but I always stop to think if the TLD actually fits the name. On some I pass on the .biz, on others I pass on the .us. Yet, sometimes I'll take both and sometimes neither.