I think that they're both lovely. I can go for either spelling; I tend to prefer the simpler spelling of a name (
Nora over
Norah,
Elinor to
Eleanor), but not always (
Susannah over
Susanna,
Sarah over
Sara). In the case of
Selah and
Sela, I think that the final "h" makes the name look more complete, but I'm not opposed to
Sela either. :)