3 models - different, but?

SGP511                                         SGP512                                SGP521

Wi-Fi 16GB                                 Wi-Fi 32GB                     LTE/4G 16GB  |white

 399£                                               449£                               499£

but what is difference - looks the same, not GB of course, or colour 

but nobody even in shop can explain me difference.

tried to read, but looks the same too - for just ordinary user.

What for i will pay extra?

Thanks.