Well that depends which vocation you are and for what you use this rings.
If you are a mage and just want to make some runes inside depot a life ring is cheaper and will regen a good amount of mana but if you are using it to hunt ring of healing is better because of faster regeneration.
Ring of healing last only 7,5 minutes but they regen 450hp and 1800 mana so if you need a faster regen it’s the best one but also the most expensive mostly rings of healings cost 1~2k (or more depending of servers)
Life rings regen almost the same amount of hp(400hp) and 1600mana but in 20min so it regens a lot slower but it’s the cheaper one (300~500gps depending on each server market) and you can also buy or loot life crystals to change for a life ring making it easier to obtain.
So it depends how fast or how much you want to spend...
(1 ROH 2k = 4 life rings 500gps)