I like both of them very much - about equally, really. They've both got a steady, strong, low-key vibe that I really appreciate and they've never been over-used.
If I was visiting here from another planet, I might actually prefer
Alan, though it would be close. Since I'm a citizen of Earth, it's
Ian all the way for personal reasons.
I would have used
Ian for my son, actually, but DH's sister had recently divorced a (very nice)
Ian so we thought it would be tactless.