Tattoos just aren't for me I guess. The very fact that you are having trouble deciding what kind of tattoo to get, and what slogan or verse to tattoo under it, suggests that you may not be 100% sure about what you want. That's a good thing.
Tattoos of course are meant to be permanent. When you get a tattoo you are effectively saying, "I am so right about this that I will never, ever, possibly want to change my mind about it. So, burn it into my skin forever." I think getting a tattoo is meant to be a "free-spirited" kind of thing to do. But, I can't help seeing it as sort of closed-minded. Leave the possibility open for yourself that you may not always want it there.
Then again, U2 are awesome aren't they? But seriously, think about a U2 bumper sticker instead.