Is a .com or a .net or a .whatever better?
Wikipedia has articles about the common domains (.com, .net, .org, etc.) that state what the domain was originally intended for (commercial, network, organization, etc.) and how they're used today. You might want to look up the ones you're considering. Also, you might want to consider using a country code domain. My email address uses a .us domain run by a family member.
ETA: A PP mentioned the possibility of people being strong-armed into giving up their domain names to companies who want it. Companies in the US rarely use .us domains, so that might make competition for the domain name less likely. On the other hand, I think a lot of people default to .com when typing a web address, so if you have domainname.us and someone else buys domainname.com, people looking for your site might end up at the .com version by mistake, much more than the other way around.