What's your riding level? A tower will get you more air, as will ballast, but you might not notice the difference if you're a beginner. The best way to get more air is to make sure your edging and standing-tall-at-the-wake techniques are really solid. Otherwise, you won't notice a difference with a tower.
Once you get higher than the point on the boat to which your rope is attached, the rope starts to exert a downward force on you, pulling you back down again. The benefit of a tower is that it allows you to jump higher before you reach that point. If you are consistently jumping higher than the attachment point of the rope to the boat, I would recommend a tower. If not, then I would say don't bother. It's an expensive purchase to make if you don't really need it. If your technique is solid, I would get some fat sacs before I got a tower.