I like Ruby, but both are lovely.
I looked up their popularity in case that matters to you, but both are about the same (Violet is #101 and Ruby is #109). That said, Violet is much trendier. In 2001, Violet was #663 and Ruby #232. Ten years later, they're about the same, so Violet is trending upward much faster.
But then, it also depends on where you live. Violet is a top-50 name in RI, OR, and Maine; Ruby is top-50 in UT, OR, and ID. Oregon loves its color names, I guess.