Because
Amy is so popular for people my age (second only to
Jennifer), I think of it as common. However, I think it is one of those deceptively unique names that seem common because there are a lot of adult bearers, but when you look at the name stats for newborns in the past few years, you note
Amy doesn't rank that high.
Here are some other even more deceptively unique names:
JaneSusanJoan - almost out of the top 1000 now
To answer your spelling question, I would use
Amy. I am not one for variant spellings. They make life difficult.