I would say as an individual, in a well designed natural vivarium, they would be happier in captivity. I only say this because I am predicting that most animals relax to a certain extent with a lack of predatory pressure.
However, serious drawbacks to the mental health of your newts might be a lack of a mate or reproductive means when breeding season arrives. As a person that prefers solitude, I can assure you I'd still be rather frustrated if I could never get laid!
On a similiar note, just being confined is a stresser. A well designed vivarium means it is large enough, and with containing barriers hidden enough, that the captive does not feel confined.
Ask me where the animals belong, and I'll tell you in the wild nearly every time. Without all this cerebral thought getting in the way, there isn't much more to life than reproducing and continuation of the species and your genetic traits.
So if this turns into a WC versus CB discussion, I vote for Captive Bred all the way. In my opinion only in rare circumstances should seasoned keepers collect animals for breeding purposes to sustain the pet trade. While I'm not exclusively innocent on this page, taking animals from the wild should not be done on a whim.