Citi analyst Mark May says that while neither are Buy rated, he prefers shares of Snap (SNAP) over Twitter (TWTR). Snap's active user base in the U.S. is more than double Twitter's, which highlights the "greater long-term opportunity for value creation," May tells investors in a research note ahead of Snap's Q2 results today. He adds that Snap is growing its active user base "significantly faster" than Twitter's and that Snap's average user is "significantly more engaged" on the platform. May has a Neutral rating on Snap and a Sell rating on Twitter.