Alison is the usual spelling in the UK, &
Allison is a surname. I like it too, but it's definitely dated here - I've never met or heard of a young one. It was in the top 100 from the 1950s to the 1980s and peaked in the 60s, so most Alisons are middle-aged, and older ones aren't uncommon. I'm surprised it ranks so high in the US.