For the record, I like neither term: “attaboy” has a back-slapping, good ‘ol boy quality, while “props” are what the theatre arts department keep in storage.
Meanwhile, “kudos” sounds like something that old, white men at the Harvard Club say.
Regardless, Millennials have apparently decided that “props” is a newer, cooler way of expressing approbation (approval) for something/someone.
Works for me.
Or, we could all just stick with “thumbs up!” (which has the virtue of being an instant emoticon as well).