I'm gonna go with the Eiffel Tower. It is a much more impressive structure, and you can actually get right up close to it, and even go inside, and travel to one of the observation decks near the top. The Pisa tower is roped off to keep people away from it, because of the danger it poses of falling over (at least that was how it was when I was there. Have they shorn it up as proposed?). It's a neat building, I suppose, but the Eiffel Tower is GRAND.
I agree with msali. When you were still allowed to climb up the Tower of Pisa without ropes, the tallest couple of levels were a bit scary to climb, because on the low side you'd look the edge and see nothing. There also wasn't that much space to walk between the edge and the inner wall. With Eiffel, it is a much more impressive structure and the view is way better
The Eiffel. Beautiful near and afar, in a very interesting area, primo restaurant, and hey, I for one prefer an impressive view of an impressive city (Paris) versus a nice but not amazing city (Pisa.) And I am one who managed to climb the Pisa tower before they did whatever roping off they are doing now.
Eiffel by far. The leaning tower is OK, but really not worth going out of your way to see. Really the only reason people go to see the leaning tower is because it leans. They take their photo of someone pretending to hold it up, and move on. Pisa is OK, but not really great (we were just in Pisa in September, and didn't bother with the tower or really sticking around to see anything). Paris is a great city, and the tower is really impressive. Pisa and it's tower really doesn't compare at all with Paris and it's tower.
Eiffel Tower, because you can see it, climb it, and then you have the rest of Paris to see. Pisa, you see it, can't climb it (at least when I was there), and then ... well, there isn't much "rest" of Pisa to see.
If you are up for it, it's pretty cool to climb to the second level of the Eiffel (you have to take the elevator to the top).