Sand Boa, hands down !! Very cool & easy to care for little guys. I totally adore them. It sounded as if the 3 foot tank was a borrower ??? Just in case; since your size requirement question hasn't been answered yet; your sand boa would easily be accommodated by your 15g. Good luck whatever your choice & congrats on the wonderful b-day present.
HOW TO .....
grow snakes in Nova Scotia