Fundamentally, beans have more volatility due to the market dynamics. The US and South America have alternate growing seasons and make up the most of the market. Add political and every other kind of instability to South America, and for at least half the year it seems there is a supply problem / threat of one haha. but seriously, look at the calendar spreads, especially n/x compared to old/new crop calendars in wheat or corn and you'll see why it translates into higher flat price vol. Soybeans are what wheat was many years ago. Also, just as an aside / mini rant....corn and beans have "better" movement than wheat technically because the core fundamentals still work...the delivery process isn't a joke like it is in wheat. Wheat spreads are broken and will be forever imo. Go read up on VSR and certain other "fixes". The worst thing that happened to the wheat market was CME letting Chicago wheat spreads become a joke, and then buying the only 2 other wheat exchanges that worked (KC and Minni).
As far as correlation, we trade almost exclusively calendar spreads in the grains, but also seasonally put on a fair amount of wheat/corn (exchange spread), and have in the past done corn/oats, but thats been 4 or 5 years ago.