The "U" lines are Stadtbahn lines. A mixture of tram and metro.
While the S-Bahn are suburban trains
Ticket wise it plays no role how you are getting to the Cannstatter Wasen (the fest grounds).
map Hauptbahnhof (main train station) area
map Canstatter Wasen
Canstatter Volkfest (beer festival) map 2013
U11 get's you right to the center of the Cannstatter Wasen near the beer tents. Travel time is 19 minutes. Back however only 11 minutes. As the U11 runs not bidirectional but on a one way loop through the city center.
You can however also start on the U9 and U14 and change at the stop Neckartor to the U11 (same platform). 12 minutes.
Stadtbahn network map
You can also exit at the stop Mercedes Strasse which is on the above linked beer festival map at the left end of the festival grounds. This stop is besides the U11 also served by the U1 and U2.
Or you can take the S-Bahn lines S1, S2 or S3 to Bad Cannstatt train station. Or any regional train to Bad Cannstatt. For both this will be the next stop. Travel time is just 3-4 minutes. You have to walk however a few meters to get to the Cannstatter Wasen (~ 350 m).
In the end thus no big difference time wise between the various options to get there.
Edited: 8:19 am, September 29, 2013