As neo mentioned, a vast majority of players (at least in the West) are CHL players. The U of S for example have only 2 players who have not played CHL, and they played Junior A. The U of C Dinos have a couple more guys who have 'just' Junior A experience. Also sprinkled in there are a few guys who played a year in the minor pros, figured they weren't going to make it big, then went to play CIS. A good example of this is Chris Durand, who was a 2nd round pick of the Avs, who plays for the U of S.
Most of these guys are playing/going to school thanks to CHL scholarships, where they get a year in school for each year they play. Some more information from the WHL on that here:
http://www.whl.ca/page/whl-scholarships
This makes comparison with NCAA schools very much apples and oranges. The US players are younger, but certainly their top end players may be better and definitely have a higher upside. CIS and NCAA teams do often play each other in exhibition games in the fall, with the NCAA teams usually coming out on top, but that may also speak to the NCAA teams looking for an 'easier' team to defeat early the same way that NCAA football or basketball teams actively scout for teams to schedule early in the season to rack up wins.