The Viair compressors are nice and they have a lot of different options. The main choice is whether or not you are looking for speed and quick use or a slower, but constant use. Most lower duty cycle compressors put out a higher CFM (air flow), but can only run part of the time. For example, a 25% duty cycle compressor can run for 15 minutes of every hour. A 100% duty cycle compressor can run for the full hour. I recommend going with at least a 30% duty cycle and personally I use a 100% duty cycle compressor. I'd rather wait a little longer to air up than worry about overheating the compressor on larger tires.
The 300 series compressors would be a good fit for 31s. The 325C is a 33% duty cycle and the 330C and 350C are 100% duty cycle. The 325C puts out 1.77 CFM, the 330C 1.06 CFM, and the 350C 1.48 CFM. The 400 series compressors are quite a bit faster, but also more expensive. Air tanks can always be added at a later time for air tools or to get a quick jump on filling your tires before the compressor kicks in.
