During the Ill met by moonlight quest I have to talk to Sinding but he's in his cell away from talking range. How do I manage to talk to him?

Had the same problem just a couple of minutes ago. I even rested about 20 times before his cell but he would just stand there against the other side of the wall.
What i did afterwards was reload a save file before entering the jail and then he walked towards me and i could start talking to him.

Thanks, it worked! Now which is the better reward: the ring or armor? effects would be appreciated

Saviors Hide-
28 Armor
6 Weight
15 spell resistance
50 poison resistance

As for the ring it lets you turn into a Werewolf when you equip it (this is unconfirmed, there are a couple of different answers for what the ring does).

If you go into your Powers menu you can select the ring as a power, which lets you turn into a werewolf whenever you want.