It all depends on how you use it. In my case, I use 3G on my iPhone, as i bring it everywhere and I have Internet when I need it. For my iPad, I mostly use it indoors, and wifi is always available to me so I didn't buy the 3G model, the iPad is also used in my car for music, loved it when iPod is on a huge screen.
My dad uses the 3G as he has no wifi at work.
So it depends on how you use it, I suggest if you don't have wifi then get the 3G model.