Messed with this decision for weeks myself and ended up with the ARB twin compressor. Expensive yes but has the highest output on the market at this writing for onboard air. I air up 37x13.5 r17's from 8 to 30PSI in about 2 minutes per tire. Not as fast as CO2 of course but, I can stay on the trail for days and don't have to worry about the bottle running out. Also it doesn't have to be checked, serviced and paid for before any trail run as it is already in the Jeep and doesn't require any servicing. Out here a fill is 20 bucks per.
Each system has its advantages, just have to decide on which end you want to pay for it.
CO2 is a faster fill but, you have to get it serviced about every 3rd fill of tires, also have to check your air pressure the next morning as the CO2 molecules will leak out and thus cause lower tire pressure. Its not much but for us OCD people it will drive ya nuts. Then there is the 20 plus dollar service fee every time you fill and you have to find a place that services the bottle and of course you are subject to their hours of operation. Also must be strapped down or mounted then you have to lug the bottle around from tire to tire, re strap down.
Onboard air... not as fast but ALWAYS there. One sunk cost and done. No maintenance, no fuss, get in and GO! Since it is mounted on board you don't have to pull it out and lug it around, one hose with a fitting and a TP gauge is all you need to get out of the back.
Since I consider myself a high speed low drag kinda guy, went with the onboard air. Once less thing to think about and manage before a run.