Since the NCAA Tournament field was expanded in 1985, the 8 vs. 9 matchup has been a toss-up in the first round. In fact, the record over the 136 games since then stands at exactly 68-68. On top of that, over 60% of the games have been decided by fewer than 10 points, making it one hell of a matchup from an entertainment perspective.
As for deciding which way to go in this matchup, it is really more about the 1-seed in each 8 or 9-seeds respective bracket, meaning you need to decide whether or not you plan on advancing them past the second round. Historically, 9-seeds have not done well in the second round, losing 62 of 69 games played. Of the seven teams to make the Sweet 16, four reached the elite eight, and Wichita State even made it to the Final Four in 2013. Of course, last season saw two 9-seeds advance past the 1-seed in their bracket to make it to the Elite Eight.
With that being said, 8-seeds have had much more success beyond the first round. The most notable instance is when Villanova shocked the world, defeating powerhouse Georgetown in the 1985 National Championship. The Wildcats were an 8-seed and remain the lowest seed to ever win a title. In 2014, an 8-seeded Kentucky team squeaked by 9-seeded Kansas State in the first round before making a run at a national title. Unfortunately, they lost to another low seed in Connecticut, who was a 7-seed that year. Butler also defied the odds in recent memory, making it to the title game in 2011 before losing.
In terms of betting, the last eight tournaments have seen 15 of 32 No. 8 seeds close as underdogs. In those games, the 8-seeds finished 9-6 straight up and 10-5 against the spread. If you go back even further to 2000, 8-seeds have finished 18-10-1 against the spread as underdogs, so keep that in mind when looking at the opening lines this March Madness.