Well compared to the envenomed cloak DR 5/- is better. After more than 4 hits before a heal it has payed out, and I hope you can take more than that before your hp gets critical. Doesn't count for spells of course, but still. (And as i understand your last reply you will have the set bonus even without the cloak).
As for greensteel as always it depends on your item. But common for greensteel items is they are made specially for the character and therefore contribute in a way that is not matched by any other item. So in this case I'd go for the greensteel.
In short:
Greensteel > DR 5/- > 20 hp
EDIT: If you play with DR 5/- at the moment and you rely much upon it you might have to get used to not having it. But that shouldn't be much of a problem.