I use both Betterment and Wealthfront because I wanted to try them out. Initially, Betterment had the better interface. However, Wealthfront has been catching up. With the recent addition of Path by Wealthfront, you can now see they're catching up with interactive tools to visualize your financial health.
Anecdotally, my Betterment portfolio has consistently beaten my Wealthfront portfolio by 2% overall. Both are set up similarly (Risk 9.0 on Wealthfront, and 90/10 for Betterment).