Speaking only of the endings, I think that both of these stories are effective. At least, both of the endings make me happy. I think that both stories ended up in the ways that the characters deserved them to end.
As far as which is more believable, I think that "Gift" is more believable. I think this is just because it relies on fewer things. All that has to happen for it to come out right is that Jim has to act as a good husband would.
By contrast, for Mary to get away with what she did in "Lamb" takes a lot more luck. She first has to kill Patrick with one blow. She then has to keep her head enough to set up her alibi and to deal with the police. Finally, the police have to not be too suspicious.