If you have narrowed down to one of those two, and are confident of your decision, I say Shellie May. You'd be unique and possibly a trend setter. I'm sure there are a lot of people out there with Tinkerbell.
I have a plan to get a Duffy & Shellie May tattoo, but I'm keeping the actual idea I have a secret. It's going to be a little bit though, because I have a tattoo I'm going to have covered and I want to do that first. But always make 100% sure you are getting what you really want! Make sure it means something to you and don't let anything anyone says change your mind. It's your body and you will have it forever. You don't want to look at it later and regret it either way.
Yeah that tattoo looks... bad. Mickey looks scary.
I'm personally not a fan of tattoos. I don't have any and I don't want any. I just don't see the point in marking my body forever like that. Plus... NEEDLES... 'nuff said (I hate needles). But it's all personal preference, so if you REALLY want a Shellie May tattoo, then go for it. Like others said, make sure it's something you REALLY REALLY want because it will be with you for the REST OF YOUR LIFE (unless you can afford tattoo removal, which from what I've heard is very painful and expensive). Good luck!