Whats the difference?
In a nutshell they are both exactly the same the only difference being that the 4G versions have a sim card slot allowing the customer to put a sim card in and being able to use the device on the Internet wherever they want without being restricted to WiFi networks. This additional feature is along sode WiFi rather than instead so if you are on a WiFi area you will be able to connect.
what should I get?
It totally depends on your lifestyle and where and when you will be using the tablet. If you do alot of commuting and travelling and intend on using it on the go then the 4G version will be better for you also if you use your phone for work purposes or have anything stored via cloud services then being able to access them anywhere would be an advantage. If you intend on mainly using it at home and you have WiFi then you may opt for just the WiFi version to save some money. It also worth checking coverage in your local area and areas you often visit. At the moment EE cover over 80% of the country with 4g and over 98% with 3G but it is worth checking
So what's the price difference?
It varies but on average you are looking at anywhere between £30-70 difference so its really all about working out whether it is worth it for your personal needs.