Here's some more detail on the difference between the USHL (tier 1), NAHL (tier 2), and the various tier 3 leagues.
http://www.ushl.com/juniorhockeyus
The USHL being the best is the primary feeder for Division 1 NCAA college programs. Those are the best programs where kids get free play, scholarships to school, and if they are good a crack at high level pro leagues. The NAHL can still supply D1 schools but it tends to be older kids who took longer to develop. 20-22 year olds. It will also supply D3 NCAA colleges (there is no real NCAA D2 league play). They offer free play but no scholarships. Kids may get a chance at lower pro leagues. Tier 3 junior programs typically tend to feed ACHA college programs. The ACHA is generally a lower step organization compared to the NCAA. Kids predominantly have to pay to play there and pay for school. A crack at low level pro leagues from here is rarer but not unheard of.
Here's a very recent article comparing the various leagues ability to develop players. At this level you really want to evaluate a team more than the league as each varies in how well it is run. Since you are paying to play you can shop around as to what team you'd like to join.
http://thejuniorhockeynews.com/?p=45169
As for comparing a swedish league to here I unfortunately have no idea. I'm also less familiar with the Canadian leagues...