The pathetic 3G services for devices such as the iPad are as limited as the cell phone plans. i.e. $30 a month for a pathetic 2GB a month. It is a joke, there is absolutely no point (in my opinion) to get a 3G data plan when the companies that provide it stiff you with such a small limit.
Though it is important to note that you can switch from 3G to Wifi when you're in your home and avoid using your plans data.
As far as size goes I think you should go for the 64GB. If you're considering it its better to get it rather than buy the 32GB then realize its not enough space.
If you buy the 32GB model and need 64GB you'll end up paying more than the $100 difference between the two because you likely wouldn't be able to sell the 32GB for as much as you paid for it.