I got the 3G version for two reasons: first, I wanted to make sure I had the option of being able to access from anywhere should the need arise, and two, the 3G model has GPS, which is very useful for a variety of applications.
Now, keep in mind that in the six months I've owned my iPad, not once have I had a need to use the 3G. I travel quite a bit, and always take my iPad with me. Just goes to show that you can easily find Wi-Fi access pretty much anywhere nowadays. So if you don't foresee yourself ever being out of range of Wi-Fi, then save yourself the money and go with the Wi-Fi model.