It's good that you're thinking about your domain name before you purchase it. Technically it doesn't matter but, from a marketing perspective, it's best to stick with a dot com domain. People are used to dot com and it's also most common.

Most business owners who have dot biz domain names will tell you they've had to work extra hard to get people to remember their domain names and they would change it if they could.

Domain names are not the time to go against the grain -- unless you're willing to spend the extra time and money on the marketing.