90% of the united-states are familiar with .com but not the world. Here in canada .CA is king. In russia .RU is king, In germany .DE is king Also there is more ccTLDs registered then .coms word wide so you can't say 90%. USA is not the center of the universe.
Dot com, that's a no-brainer. People are just too used to it to choose anything else if possible. Think about it, when you hear of a new service what do you type in automatically? Twitter.us? Twitter.net?
If you're going for page on of Google SERPs, I believe studies show that .com is king followed by .org, and then .net.
I always grab .com if it's available, but I'll take a .net if it's not (especially if there's nothing on the .com anyway).