Well my.02 is dont' do it. I've never seen a tattoo and thought anything else other than graffiti on a work of art. It is so sad to me when a beautfiul woman has drawn all over themselves and ruined perfection.
Do it if you must but if you want to make a statement - get a tshirt. Leave your body as attractive as possible. I've never seen a tattoo and thought 'wow she's more pretty'. I generally spend my time thinking 'I wish that wasn't there'. Even little butterflies on your ankle. Still don't like them.
It is all your opinion, you are the one who will have this on you, but I like the one on the back. But, if you want it to show a lot more, I say just go ahead and just go for the arm one. It will look really cool on the shoulder too.
Go with your back. It's also easier to cover. Some workplaces are still very strict when it comes to tattoos and such, and it would be a shame if you lost a good job opportunity to someone else just for a tattoo.