Carbon filter all the way. I've seen quite a few DIY for making filters, i can't attest to how well they work but none of them come close to the 10-20 pounds of activated charcoal you can get with a "real" carbon filter. I guess it depends on the # of plants and how stinky they are.
FYI you don't have to actually vent the air anywhere, you can just filter it back into you grow room...and keep refiltering it really.
Ona works way better than a regular air freshener but it still tends to smell like ona and weed.
Cheers,
Silverserf