.ca by far is better. You can say what you will about .us be the fact is most americans don't identify with .us they identify with .com .us is like a collection extention if anything for corporations who like to cover all extensions sure you may have seen x company use .us but I would be willing to bet they own the .com too. so they're free to promote their business on their .us extension.
.ca is much more excepted in canada, companies are embracing it. Sure it's true alot of generic .ca's are owned by certain individuals but you know what I bet they're getting offers weekly but are waiting for the offers to hit higher figures.
US is no sleeping giant it's a failure. With a couple of noticable sales every few months. .ca is the beast roaring ccTLD 2008 and will be the clear winner.
The only thing that needs to happen for .ca to really explode is to drop the restrictions and let the rest of the world get a taste for our north american friends.
I think .ca has more potential then .us but there are better ccTLD then .ca
It's not that there's better, it's that the other ccTLD are open to all. So only a handful of canadian domainers are cashing in silently and believe me they are. If you're canadian and you can grab some nice LL/LLL or generic domain either hand reg or tbr do so. It will be a good investment.