What is the difference between the John Deere STS16 and STS20?

Here is how the John Deere STS16 and John Deere STS20 compare on John Deere's published specifications:

• Horsepower: John Deere STS16 400 hp vs John Deere STS20 400 hp
• Fuel capacity: John Deere STS16 135 gal. vs John Deere STS20 135 gal.
• Wheelbase: John Deere STS16 180 in. vs John Deere STS20 180 in.