There is no real difference. However a .US domain will more likely be geotargeted for the United States, so if your audience is international, chose the .com/.net.
A .com is better for branding since everyone associates a website to a .com. In terms of SEO, the .com is also better because Google does not have a separate site dedicated to US listings. Therefore, although having a .us would be the same as having a .com for Google global search, it will actually hurt you if you want rankings and traffic from country specific search. e.g. you will not do well in google.co.uk with a .us domain. You may think that you don't need international traffic, but a lot of visitors to my sites actually come from other English-speaking countries such as Great Britain and Australia.
exactly what KC Tan said. extension doesnt matter when it comes to the se's. I developed a few .us sites and they had no problems ranking in the SERPs.
I thought about this a little more. I'm not sure this is entirely true. If one of your keywords happens to be that tld, then a domain with that tld will rank better. For example, my wife has a mobile DJ company. I registered her domain name in Djibouti so I could get the .DJ extension. Sure, its a small pond that should be fairly easy for any DP member to dominate, but her site ranks very well for a lot of generic DJ terms, which is amazing once you consider the amount of money we've spent ($0) and its narrow geographic focus.