You can get tablets that are 3G enabled and take a sim card exactly the same as a smartphone.
You can get PAYG sim cards that have a data allowance, this is necessary to use the internet without a wifi connection.
It really is a matter of personal choice. I have a smartphone and a tablet and find the smartphone far more convenient for emails and simple browsing. My good friend has the Moto G phone which is around £100 and he highly recommends it. In the past he has had expensive iPhones and Blackberries, so it must be okay.
http://www.gizmodo.co.uk/2014/04/a-motorola-moto-g-for-99-99-is-your-best-budget-handset-deal-of-a-day/
You need to find the cheapest provider for PAYG with mobile data.