We Did ASA Northern in Indiana last year. Those folks know how to run a tourney. We had a good time, good competition. We haven't played in KC so can't compare the 2.
My older daughter has been to both and enjoyed both immensely. The World Series is a longer tourney ~ we were there from opening ceremonies on Monday night until Sunday. Northern Nationals run a bit shorter, so that might be a factor depending upon if families have sufficient time off. Both were very well run. Sorry this isn't much help as far as saying "go to this one", but I think you will enjoy either one you go to!
We (the team/parents) decided on the USSSA WS. Played in ASA state tourney yesterday and today, and play in U-Trip state in 2 weeks. I think next year, which will be daughters 2nd year in 14U, she absolutely needs to be on a team that pushes for ASA Nationals, no matter the cost.