Golf,
It really depends on your intended usage. I went through the same itteration a few months ago. (primarilly as I am running temporarily without a spare, while I make some revisions to the bumper and spare mount).
Options:
Portable compressor - cheapest, can transfer from vehicle to vehicle, no air tools, slowest, infrequent use OR short, lifespan takes up cargo space
CO2 tank - depending on brand and size, your next cheapest option, or equal to OBA. Can power air tools, refilling costs, can transfer from vehicle to vehicle, high use and long (near infinite) lifespan, takes up cargo space
On Board Air - depending on brand and size the most expensive option, With additional tanks, can power air tools, stuck in vehicle, variable lifespan depending on brand and usage, does not take up cargo space.
Portable Compressors are a cheap option, and seeing as that is the benefit, I wouldn't drop big cash on top end models
I purchased a Superflow, and it works, run under $70 and deos what I need it too.
CO2 tanks have a few options
Powertank has the best reputation, and runs pretty expensive. The tanks need to be refilled, rather than swapped out, so plan additional time when filling.
Smittybilt has a questionable repution on a number of their other products, but I am really not certain about air tanks. Compressed gasses is fairly well regulated in this country, so I would think that the Smitty tank must meet some minimum standards as far as regulator sealing and tank safety. Smitty only has the one size, so make sure that works for you.
ALSO, there is a guy on ebay selling home made units similar to powertank with standard airtanks. The benefit is being able to just swap out tanks rather than refilling, which may be a benefit depending on your filling facilities. His prices were similar to a smitty system. I didn't like the fact that he didn't have great protection around the regulator.
On Board Air.
This is going to be far to expensive unless you are able to plumb the system yourself. It is not rocket science, but time consuming and some head scratching. Compressor, lines, additional circuit, and storage tank. To many variables to consider here as you are building it to suit your needs. Viair and ARB offer products. Soem people swap out AC compressor for an On Board Air system, but I personally like AC.