The best advice is to reg both .biz and .us if all other extensions are taken and the name is worth regging in the first place. .US has special appeal as a known country code and imo works very well across many different domain categories and business sectors.
.biz has a very good future and will gain increased attention as people continue to take business online and look for alternatives to awkwardly worded .coms. It's a sleeper with a unique identity. For example, I have domains like AirConditioning.biz, MiamiRealEstate.biz, NewOrleans.biz (high interest, narrow focus) which get clicks, receive offers, and are good fits for the extension. Business, finance, professional services, and retail categories (jewelry, furniture, cars) are a nice match too.
Don't forget .info. .info is a great extension and continues to be a super logical namespace with really good user recognition. I like all three extensions as each suggests its own emphasis.
A principle of internet growth that seems overlooked by many domainers is how quickly a trend can develop and spread on the net. While it's not an everyday occurrence, any extension can gain a quick base and significant momentum when a known athlete, star, or company utilizes it. By the time that trend is clearly visible, it's way too late to enter the game. Case in point are the multitude of three word .com names that were passed over repeatedly ... and that now go for 4 figures routinely. You couldn't convince most people of their value 5-6 years ago. This is the benefit of forward thinking and calculated risk/true speculation.