All iPads within the same product line share the same platform. That means, that means you get the same performance regardless of the built-in storage flash memory capacity and the 3G/4G modem.
But what exactly are you missing by investing in the cheapest iPad 3 16Gb without 3G/LTE modem?
The first thing is obviously the storage capacity. Out of 16Gb, only 13.3 is available for the user. However, the difference in price between the 16Gb and the 64Gb models does not represent the actual difference in flash memory cost: it’s a rip-off! Moreover, even 64Gb is still not enough, especially if you are used to 500Gb+ HDDs in most modern computers. So either way you will have to backup and copy over something, or use a network storage at home. For jailbroken devices, there is an option to use an OTG Cable (Camera Connection Kit) to use external flash cards and USB sticks and drives on the iPad, so investing in overpriced device capacity may be not a very good idea.
The next thing is the 3G/4G LTE modem. This one is for cellular data. However, you cannot make voice GSM calls, as the modem inside the iPad is data only. So you still have to carry a cellular phone with you. If so, why even bother purchasing another SIM card and data plan for the iPad, if you already have a cell-phone? Just use the phone to tether 3G/4G internet over Wi-Fi: plenty applications exist for major mobile platforms (including Android and Symbian) to do that. Moreover, the iPad uses North America specific LTE frequencies, so if you don’t live in the States or Canada, getting a 4G iPad is just stupid: it won’t work in your country.
Another thing that a Wi-Fi only iPad lacks is the GPS functionality. It turns out, the cellular modem chip also incorporates the GPS chip, that means no 3g=no GPS on the iPad. However, iPad uses assisted GPS, that means it need data connection for tolerable navigation performance. With 3G turned off, the device spends so much time acquiring satellite data, that makes in unusable. So the performance of GPS has nothing in common with that 1-minute position acquisition you maybe familiar with since SirfStarIII GPS chips of the mid-2000’s Windows Mobile communication devices.
However, Location services are still available on the non-GPS iPads through Wi-Fi: the device uses triangulation from known Wi-Fi spots locations database to determine your position. Much like the iPad’s assisted GPS, this won’t work in the deserted unknown areas where you normally need GPS navigation in the first place. So you get decent navigation experience on any iPad, you’ll have to invest in a bluetooth GPS device to pair it with the iPad.
So, it turn’s out the cheapest iPad is not such a bad investment after all.