I'm not one for different spelling of name that sounds exactly the same (
Allison v
Alison). I also don't believe in giving a nickname to a person when an actual name could be shortened whenever and then revert (It's easy to go from
Elizabeth to
Liz to
Beth and back to
Elizabeth. It's hard to go from
Ally to
Allison).
I personally like the name
Allison, and compare it to names like
Briana. Sporty yet feminine, a tiny bit dated but too much, and easy to shorten and give nicknames for.