Just a brief report on the data use for my iPad 2. Because I need internet access for my laptop when I'm traveling and there is no wifi available, I have a mobile hotspot from Verizon (Samsung SCH-LC11). Upgrading from a previous netbook I paid $49 for the mifi unit, about the size of a playing card and 1/4 inch thick. (I upgraded at the end of a contract so I received a $50 credit. Otherwise, the hotspot would have cost me $99).
I pay $50 per month on a two-year contract for 5 gigabytes of downloaded data with an additional $10 per gigabyte up to $80 per month for unlimited access. Since I have the hotspot contract in place, it made no sense to pay for an additional 3G data plan for the iPad. (I don't happen to need GPS functionality in the iPad, but if I did, I could have that functionality in the wifi iPad for an additional $100 purchase of a GPS unit that connects to the iPad.)
Had I purchased a 3G iPad, I would have paid an additional $130 and could have had a 2 gigabytes per month contract for $30 per month with the same $10 for each additional gigabyte, but no $80 ceiling. Further, I could turn off the 3G coverage for any 30 day period I didn't use it at all.
I've never used more than 5 gigabytes per month on the mobile hotspot since I'm typically at my home where I use the wifi network there. But in order to get an idea of how much data I would actually consume with the iPad I've left it connected to the 4G/LTE Verizon network for much of the last couple of weeks and tracked the amount of data consumed and the performance of the mobile hotspot.
On the performance side...
() My iPad typically shows downloads between 6000 and 10,000 kb/p/s(Kilobytes per second) with occasional speeds of up to 14,000kb/p/s when I'm in a 4G/LTE service area. This is approximately 3-8 times faster than the equivalent performance of my 3G smartphone at the same location.
() I should note, by the way, that while the 4G/LTE performance is much, much faster than 3G, it's not usually nearly as noticeable as the performance numbers might suggest. Browser performance is noticeably snappier and lags from buffering are almost non-existent on 4G, but for streaming audio and video, 3G speeds are usually acceptable. Likewise for web browsing. Still, 4G performance is addictive and the more I use it, the more impatient I am with anything less.
() It also bears noting that 4G is a significant battery drain on the mobile hotspot. Running on battery continuously connected to a 4G network will exhaust the hotspot in about 4-5 hours. Connection to 3G will get another 1-2 hours of battery life. Neither of these figures is especially impressive compared to an iPad, but I usually have a wall socket handy when I'm using the hotspot so it's irrelevant in those cases.
On the data use side...
() I've done occasional video streaming, updated about 30 apps, and almost continuous audio streaming for about 10-12 hours per day over the couple of weeks. Total data usage is nearly 2 gigabytes for the period. Extrapolating that usage for a full month, I would use about 4 gigabytes per month if I relied exclusively on the mobile hotspot for the iPad. Since I wouldn't exceed the 5 gigabytes maximum for my data plan, the cost would be the minimum $50. If I were using an iPad data plan, however, the cost for the 4 gigabytes would be the same ($30 base price + $20 overage.) And if I exceeded 4 gigabytes by a single byte, the cost would rise to $60.
I'm not citing these figures to argue one way or the other for Verizon or for a mobile hotspot versus a 3G iPad. AT&T and Sprint prices for mobile hotspots and monthly data plans are slightly different but not by a significant amount. Likewise, Verizon's 4G/LTE network is more widespread and faster than AT&T's, but an individual user's experience may be quite different from the "average." And if a user needs 3G access less frequently (i.e. not at all for at least 30 days), the cancel/reinstate provisions of the iPad data plan may be considerably more economical than a continuous two year plan. Finally, of course, if one never exceeds 2 Gigabytes per month, the iPad data plan is also significantly less expensive.
Nevertheless, if one wants 4G service, these figures should provide some guidance.