The figures you reference are the amount of solid state storage (in gigabytes) that model iPad has installed.
The higher the number the more storage available to you to store content locally on the ipad. Content consists of but is not limited to applications, pictures, movies, music, documents, etc. Applications in some cases exceed 1GB but most are reasonably sized.
Your decision should be based on how much content you will want stored locally on the iPad bearing in mind any movie or other large object need not reside on the iPad permanently but rather only when desired or required.
I had a 64GB original iPad and presently have a 32GB new model and initially feared it wouldn't be enough but with the combination of cloud services and home sharing I've come to realize 32GB is ample room for my needs.