I would suggest a 6 gal compressor with a honda eu2000 generator. That is what I have and both units are lightweight and very portable. The 6 gal compressor will run air tools like impacts and ratchets. It will have trouble keeping up with things like sanders grinders and drills.
The 20 gal compressor will require a much bigger generator and if you plan to just fill it once at home and not plug it in at the track you won’t be able to run any air tools. You will pretty much be limited to filling tires.