I would think cht > dragonfang for off hand but I'm not going to try and act like I've actually done the research to back claim up. I know I won't be wasting dkp on dragonfang though.
Well you can start off by looking at the amount of AP you gain from Dragonfang through agility and realise it's not as much as CHT, and that 16 agility also doesn't give you a full crit so it has worse stats on both accord. Then there's also the lower attackspeed, which will mean less poison procs. kthxbai