Well, in the case of the Arabian sand boa, Eryx jayakari, it seems that the answer is - both.
Although many species of snake lay eggs, boas generally give birth to live offspring. In the journal Evolution, Vincent Lynch
There is a short commentary on this study in the recent edition of Nature:
Many vertebrates have abandoned egg-laying for live birth. But despite some suggestive examples, convincing evidence for the evolutionary reversal of this trait has been lacking until now.
Vincent Lynch and Günter Wagner at Yale University, New Haven, carried out a phylogenetic analysis of 41 species of boa snake using recent DNA data. The most parsimonious explanation of the phylogeny, they conclude, is that the Arabian sand boa, Eryx jayakari (pictured ), one of only two species of egg-laying boa, re-evolved this ability some 60 million years after the transition of the group to live birth.
E. jayakari also lacks the egg tooth other oviparous snakes use to tear their way out of the egg, additional evidence that egg-laying was lost and reacquired, according to the authors.