Never seen a coupon on renderotica, but I have seen offer like "artist x at y% discount", and use it. About the points, never expired for me, but maybe is because I never keep a lot, and I used them (though I don't buy often stuff on Renderotica, mostly when there is a discount on something).
Renderosity for discount points seems to me the best - though I am biased by being prime there, it gives points a bit faster, and IMO most of the free assets every month are better than Daz.
Daz now has changed a bit things, I noticed the expiration of their "tokens" for discount is longer even for "normal" users, and the premier get the free bundle a month that can be good, plus the vouchers - but as far as I remember, one year of Daz premier costs more than one year of prime on Renderosity, so as someone else said, if one buys every month some 50 (their estimate) USD of stuff, Daz premier is good, otherwise, it may not be so convenient.
The positive side is that you can do it only for one month, but that is also the negative, because the old Daz+ you could for one year at a discount, the prime on renderosity I remember I did one year at a discount (if I remember correctly), the new Daz premier is only per month, there is "one year and you get the equvalent of one -two free months".
