Tablets come in two flavors: either Wi-Fi only, or packing 3G/4G from a carrier. In the former, it can be purchased without a contract and usually for a lower price. The 3G/4G model typically costs more (although it may be cheaper thanks to a carrier subsidy, which is offset by the cost of the plan), and comes on contract. The benefit of that added radio and contract is fairly obvious - you have access to data nearly anywhere you go.
So, which do you prefer? Are you content to use Wi-Fi (whether via tethering or just a normal network), or would you prefer the added functionality (and cost) of the 3G/4G radios?