Alexandra is gorgeous and timeless.
Alexa is classic, but has a much more trendy feeling to me. I was tired of it in 1983.
Alexis is all boy to me.
As for popularity, I can't say. I have noticed
Alexandra and other female variants getting less attention than
Alexander. I'm surprised
Alexander is still so popular. It's like the housing bubble -- I keep waiting for it to burst. So yeah, I guess I do see
Alexa /
Alexandra declining in popularity over time.
Alexandra stands the best chance of still sounding fresh years from now.