Have an iPad wifi only iPad and a Verizon Mobile Hotspot 4G/LTE. Just returned from a week long business trip where the "free wifi" in the hotel wasn't worth activating. i.e. video streaming impossible and audio download choppy with much buffering.
As it happened, although the location was somewhat off the beaten track (small city in the midwest), I was able to access the Verizon 4G/LTE network for both my laptop and my iPad via the hotspot at the hotel. Not so at the client site. There, the iPad was running exclusively on Verizon's 3G network via the hotspot.
I was on the network at the hotel for a minimum of eight hours per day using mostly audio streaming with some video streaming thrown in (about 1 hour per day.) Online at the client site for about 6-8 hours per day.
I used a total of about 2.5 gigs of data over the week. Had I been using a 3G iPad, the cost would have been $40 ($30 2gigs + $10 for an additional gig). A two week trip would have run between $60. As it is, I pay $50 for 5Gigs per month so I have about 2.5 gigs left for the remainder of the month before encumbering additional charges.
In other words, for light use (i.e. less than 2 gigs), the savings amount to $20 per month for a 3G iPad. For anything over 4 Gigs per month the mifi hotspot is actually less expensive ($10 less) than the 3G iPad data plan.
(This assumes at least some use over the month, of course. If the cell network is not used at all, the iPad data plan saves $50 per month, i.e $0 vs $50 on the always available mifi data plan.)
Verizon's 4G/LTE performance was excellent. I averaged about 6-8 mps on the 4G network versus about .75-1.2 mps on the 3G network at the client site. Both video and audio were perfect on the 4G network; largely acceptable but some buffering of video on the 3G network.
In short, if all one is doing is surfing the web, handling email, YouTube videos, etc., 3G service is perfectly acceptable. Streaming content, on the other hand, especially video content such as movies and tv shows, is MUCH better with the 4G network.
In my case the mifi solution is much better than a 3G iPad. I have to have a mobile hotspot for my laptop when I travel. (I could also tether the iPad and laptop to a smartphone but only a few phones support 4G at this time.)
Thus, it makes no sense to pay extra for a separate iPad data plan limited to a 3G network.
Others who expect to use internet connectivity less, who don't have the need to connect any other devices to the internet, don't have access to or need 4G connectivity, and/or don't have the ability to tether a device with a data plan of its own with the iPad may well reach a different conclusion.
Disclaimers and notes:
() Note that all of this applies only to the Verizon 4G/LTE network. Other "4G" networks have neither the current coverage nor the performance of the Verizon network (including AT&T's.)
() As others have noted, the 3G iPad also provides GPS functionality missing on the wifi iPad. To get that functionality on the wifi iPad, one has to add a separate bluetooth GPS receiver to the iPad. Still, the price of the GPS (less than $100) is less than the difference between the iPad wifi only and 3G models ($130.)
() The notion that the mifi hotspot is a less portable or more complicated solution than an iPad with 3G connectivity built in is, at least in my experience, not true. The hotspot I have is the size of a playing card and weighs about 3 oz. Using it consists of pushing a button on the device. The iPad connects automatically.
