My advice is first seriously think about what you want to do with the iPad. Is it going to be used for games or music? What about email and calendars? Is that essential to you? Web browsing? Presentations? Do you need something that is very portable (e.g., iPad Mini), or do you need a larger screen (fullsize iPad)?
In my case, I use my iPad for work, so 3G/LTE capability is a must for getting email outside my office or home. I also like to watch movies and listen to podcasts on my iPad when I travel or excercise, so a larger capacity iPad (64GB) made sense for me.
After you've thought about how you intend to use an iPad, go to an Apple Store and play with them. Then choose the one that's right for your needs.