I prefer carbon. If im gonna add drag to a piece im gonna get something that cleans the smoke not just something that bubbles
A/C is expensive and uncessary. You buy them for looks, or if you want an extra chamber to take larger hit. They are not necessary to catch ash because this is solved with screens, mesh or glass. Theres no need to buy something that costs as much as an actual bong when you can buy a screen. So people do this for the sake of simply BUYING the A/C. Carbon filters catch ash, and keep your piece clean, as well as your lungs. Carbon filters are completely functional, A/C's are unnecessary and easily replaced with a screen, and do not keep your piece clean as well as a carbon filter. So its up to you.