If you must get a piercing, put it in your ear. That can look elegant and tasteful. Putting a piercing on your tongue or eyebrows looks weird and cheap, putting one on your belly button is again, cheap, and you wouldn't get to show it off as much. Personally, I don't like piercings outside of the ear, and earrings probably aren't the look your going for if you're getting a piercing to be 'sexy'.
If you get a tattoo, don't get it on your lower back. I don't mean to be a prude, but that pretty much instantly relays to a guy 'whore'. Some guys will totally dig that, but usually not the right kind. Tattoos at least are much more likely to be tasteful.
Honestly, my final verdict would be neither, but that's my advice if your heart is set on it.