I think sranc will be like crack to the nonman's coke. Stronger, doesn't last as long, and much more adverse effects.
I don't think chanv is sranc though. Chanv is rare, expensive, and mysterious even to those who take it. My money is on sorcerer salt. That would be rare, difficult to acquire, and thus worthy of the price. Plus we have the not so subtle scene at the end of TTT, where the boy is collecting a bit of the salt "that would be his fortune". Sranc, on the other hand, are ridiculously abundant in the north, and people hunt them all the time. If that was the source of chances then there's no reason for it to be so scarce, nor so expensive.