What air compressor to buy depends on what you plan to use it for. If you're an active tool user you might need a larger compressor than someone who uses it mostly to fill their tires. Take a look at the air usage of the air tools you have and then check the CFM of the compressors you're looking at.
I bought a DeWalt portable inflater to carry in the trailer. It's light, small, and does the job, albeit a bit slowly. I have lots of DeWalt batteries so it was a no brainer. It will inflate my 120 psi tires very well.