A tablet data plan can really help you get the most out of your tablet. All four of the major US carriers offer tablet data plans. Carriers offer plans that are prepaid, postpaid, share data with a smartphone plan, and even free data plans. Most of the tablet data plans cost about the same price per GB of data, but with all the options available it's important to choose the best one for you.
Before you buy a tablet:
Tablets with cellular radios can cost quite a bit more than their wifi-only counterparts. For example, an iPad or iPad Mini with cellular radio costs $129 more than the wifi-only version, while the Nexus 7 with LTE costs $80 more than the wifi-only Nexus 7.
Most modern smartphones support wifi tethering, which allows you to use your smartphone's cellular data connection over wifi with a tablet, laptop, or other wifi-enabled device. Tethering is included for free on some smartphone plans, while others may require an extra monthly fee. If you're the type of user that just wants to use cellular data on your tablet or laptop a couple times a month, tethering with your smartphone may be your best bet.
Best tablet data plan for standalone use: AT&T
AT&T offers the best plans for wirelessly using a tablet by itself. AT&T has solid LTE coverage and a very fast HSPA network where LTE isn't available.
The range of options AT&T has is great. The 1GB for $25 plan that doesn't expire for 3 months is a unique option for people who don't need a ton of data and whose use fluctuates from month to month. The only downside to AT&T's plans are that only the $50/5GB plan and non auto renewal plans support wifi tethering.
Straight Talk is a prepaid carrier that runs on AT&T's network that has tablet plans with $15/1GB and $25/2GB available..
Best tablet data plan for road warriors: Verizon
Verizon has the best nationwide LTE coverage and wifi tethering is included on all plans, which may be important for frequent travelers. They offer both postpaid and prepaid tablet plans. The postpaid plans require a 2-year contract (for which you'll get a $100 discount on a new tablet). The prepaid plans can be canceled or renewed at any time, but you won't get a discount on the tablet purchase.
Free tablet data plan: T-Mobile and FreedomPop
These two plans offer a little bit of free data every month, and you can upgrade to a paid plan if you'd like. T-Mobile offers 200MB free and FreedomPop offers 500MB. FreedomPop runs on Sprint's network, and neither of these carriers has the best coverage outside of large metro areas. However, if you live in an area with good coverage and don't need much data then check out these plans.
FreedomPop is currently offering refurbished iPad Mini's for $300 that will work with their plan. If you upgrade to a paid T-Mobile tablet plan it will include unlimited 2G data after the 4G allotment is used, plus music streaming from Pandora and iHeartRadio won't count against your cap.
I'm currently using T-Mobile's free data plan now that I don't need a lot of data. I have no complaints other than I wish the coverage was better out here in the suburbs.
Share smartphone and tablet data
AT&T's Mobile Share and Verizon's More Everything plans allow you to add a tablet to your smartphone plan for $10 per month. However, both plans already include free smartphone tethering, so before spending the extra money on a cellular tablet and an extra $10/mo be sure you really need it.
What about Sprint?
Sprint doesn't stand out from any of the above options on pricing or unique offerings, and their coverage isn't as good as AT&T or Verizon.
Which tablet to buy and how to switch?
Many new tablets are sold unlocked and will work with multiple carriers. For example, the iPad Air and iPad Mini with Retina are unlocked and a single model will work on all US carriers, see more info here. The new Nexus 7 is sold as an unlocked GSM device, so it will work on either AT&T or T-Mobile in the US.
If your tablet supports it, it's easy to switch between carriers. Just get a new SIM card and activate service. I've used my 'Verizon' iPad Mini on Verizon, AT&T, and T-Mobile. It is the original iPad Mini so it doesn't support LTE on AT&T or T-Mobile, but still works on '4G' HSPA. AT&T doesn't charge for new SIM cards, but T-Mobile charges $10. Make sure you get the correct size SIM card. Most new devices come with nano (iPads) or micro (Nexus 7) sized SIMs.
So rest easy when trying to decide what model iPad or Nexus 7 to purchase. Perhaps go with T-Mobile since you'll get free data to start with and save $10 on the T-Mobile SIM if you ever wanted to get one later. If you see yourself using more data and don't want to worry about acquiring a SIM, just purchase the device from the carrier of your choice or from the Apple Store.