This is just my opinion, but I know that I would regret not getting the snake I originally wanted (to me, that would be the red tail boa in this case). I develop strong emotional bonds with my critters, but I realize that not everyone else does. If you can "switch" from one snake to the other that easily, then change your mind. But if you're here asking advice, then I suspect you are torn about the idea of leaving "Ivy" behind. If that's the case, you'll regret not getting her.
Cinnies are great, but they are available and they aren't too outrageous in price either. To be honest, I would suspect that any cinny found in a "shop" might not be as high a quality as you might find from a good breeder (of course there are exceptions!).
But you will always be able to find a cinny - whether you've seen them around or not - they are available and always will be. But Ivy is Ivy and she won't always be there. Just my .02............