One thing our family had to consider was responsible usage of the shared data plan. My mother-in-law didn't even know how to log into the Internet, so she obviously didn't use much data. My wife limited her usage to times when free Wifi wasn't available or during emergencies when the power was out. It was another family member who routinely ran up the bill due to data plan overages.
If you have a family member who is likely to play a lot of online games or surf the web while out of Wifi range, then look for the family plan that offers the largest data plan for a reasonable price. Otherwise, someone in the plan will want to monitor the data usage of all other members.