I get asked this question a lot so thought I would give you the best answer and that is that I can't tell you, you have to decided but I will help you understand which one will suit your needs.
The first thing to not is that if you have an iPhone you should buy a WiFi only iPad because with the next software release you will be able to use the internet from your iPhone on your iPad
There are two questions you need to ask to determine which iPad to buy. First - Do I need to use the internet on the go? If you answer yes to this question you will need a 3G iPad if No that you will need a WiFi only iPad.
I know a lot of people that have bought WiFi only iPad to just use at home on their wireless network. This is a great option if that is what you want it for.
Secondly, how much space do you need? If you are not into putting music and movies on your iPad then get the 16GB, but if you are like me and want everything on your iPad then get the 64GB.
The second question is really important because it you go too small then you will get frustrated. I always suggest that you buy a little bigger than your current needs because there is always the chance that your needs might change.
My personal recommendation is if you own and iPhone buy the WiFi only 64GB and if you don't have an iPhone then get one!