They are all EXACTLY the same EXCEPT the memory it can hold is different, 16 gb is a lot but is least of the 3. Then the 32 which a ton, and the most memory is the 64 gb which you won't need all that memory unless you put about 30 movies on it.
They differ in the memory size. 16 giga bytes, 32 giga bytes, and 64 giga bytes. The more the bytes, the more data your ipad will be capable to hold. (Data: applications, music, pictures and other files of this sort)