For me, my iPhone is with me virtually 24/7, so it makes more sense to just get a Wi-Fi iPad without the 3G and use my iPhone as a Wi-Fi hotspot if necessary. This arrangement makes sense from a financial and logical perspective.
I am with AT&T, so I pay $25 a month for 2GB of data on my iPhone 4. If I had an iPad with 3G, it would cost me another $25 for a separate 2GB of data on the tablet. For $45 I get 4GB of data shared between the two.
Not only do I end up saving $5 a month, but it gives me more flexibility in case I use more than 2GB of data on one of the devices--like 3GB of data on my iPad, and 1GB on the iPhone. Using separate plans, I would end up paying $35 for the iPad ($25 for the 2GB and $10 for the additional 1GB), and $25 for the 2GB of data on the iPhone 4 even though I only used half of the available data. In this scenario, I would end up paying $15 more for the same 4GB of data just because it is divided between two plans.
16GB vs. 32GB vs. 64GB: How much space is enough? One of the rallying cries and primary selling points of rival tablets is the availability of SD memory card slots and USB ports to allow for expanding storage beyond the internal capacity. The iPad doesn't offer that (although there are ways to do it using the camera connection kit and a little hack magic).
You can rely on online data storage services like Box.net or Ubuntu One, or you can use a device like the Seagate GoFlex Satellite wireless external hard drive. But, if you don't want to carry the extra gadget, and you don't have a 3G iPad, and you need access to data while on the go, you will be limited by the internal storage capacity of the iPad.
Personally, I recommend the 32GB model for most people. 64GB may be overkill for a device like a tablet. With the various online data storage services--including the upcoming iCloud service from Apple--there really isn't any need to turn the iPad into a massive portable data storage unit.
That said, 16GB can fill up pretty quick. Between the music, movies, or books you might store on the iPad, the various apps you use, podcasts you watch, and photos or videos you record, with only 16GB you might constantly find yourself keeping one eye on the available space remaining. Get the 32GB--you'll thank me later.
Just for the record, for the purposes of this project, I am using 64GB WI-Fi model of the iPad 2. So, keep that in mind as you read about my experiences over the rest of these 30 days.