Hey, With united airlines preferred seats (https://rightcheckin.com/blog/reserve-your-seat-today-with-united-airlines-seat-selection/), you can enjoy added comfort without having to pay for a complete upgrade. They are typically arranged close to the front of the economy section. Based on my experience, it was easy to choose a preferred seat when checking in online. Having extra legroom made my trip considerably more comfortable. If you want a little more space without paying for premium classes, it's a good option.