I have the onboard compressor for the spare but a C02 tanks always trumps a compressor. if I get a puncture, I can plug it and fill from the tank faster than a compressor can fill the tire.
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Having had several OBA systems(Yorks, sandens, Kilby's stuff, 12v compressors, etc), CO2 and the built in Cayenne OBA(not the little compressor they give you without air suspension) I'd disagree that CO2 is always better. I have a 10lb Powertank and love it, but....... It needs refilled, it takes up space, you have to remember to bring it and the hose, has to be secured properly, which usually means the bracket takes up space even when you leave the tank at home, its eye candy for thieves, etc.
So much nicer to just have built in, tucked away, always there OBA! The build in system with the air suspension Cayennes have TWO reserve tanks, the compressor is built into the passenger rocker and the air chuck port is in the passenger footwell, with a hose/gauge under the rear seat. Pretty slick and powerful system. Probably couldn't keep up with air tools on the trail, but I'm probably not going to have them with me in the CTT anyway!