iPad wifi only and mobile hotspot here. There are costs and benefits with either choice. I use a fairly powerful laptop on the road and must have an internet connection even if the hotel or customer site doesn't offer it. With a mobile hotspot I get 4G internet connectivity (almost as fast as my cable modem at home) for my laptop, iPad, and my smartphone where Verizon's LTE is available. (It usually is where I travel.)
Saved $130 on the price of the iPad and I get much a much faster internet connection (up to 10 times faster) than I would have with a 3G iPad or a 4G tether to a smartphone. Upgraded a netbook from Verizon so the cost of the hotspot was only $50.
(Note: Dedicated wifi hotspots are, as a rule, much faster than tethered smartphones, even the 4G variety. And the 4G LTE Verizon network is much faster than other "4G" networks. I routinely get about 10,000 kbps or more from the hotspot in a 4G area.)
The downside is that I don't have the month-to-month $30 data plan. I pay $50 for a five gig per month plan (which I literally never exceed) on a two year contract. But since I would have to pay that for my laptop anyway, it's a sunk cost.
There is also the issue of a battery charge with the hotspot. My iPad reliably runs for 8-10 hours on a charge. The hotspot is about half that. On the other hand, I've literally never needed more than four hours of internet connectivity in a location where I can't recharge the hotspot. (When camping I have a mobile power source that serves the purpose.)
In addition, keep in mind that your iPad will consume power considerably more quickly if the 3G radio is turned on. So you're sacrificing some of the iPad's excellent battery life for that 3G connectivity.
All in all, I'd say that if you're interested in taking advantage of 4G internet connectivity, especially that offered by Verizon's super-fast LTE, go with a hotspot. You won't be able to "upgrade" a 3G iPad to 4G. Period. But be aware of the considerably higher monthly data charge, especially if you don't need 3G connectivity on an ongoing basis. Consider the battery consumption issues for each device and as noted above, keep in mind that they cut both ways.
On the other hand, if your iPad is the only device for which you need a data plan, find 3G connectivity to be sufficient for your needs, and/or often don't need it at all, and need the GPS functionality provided with a 3G iPad, go that route.