Pretty much in all cases WhatsApp is cheaper.
If you have Mins and Texts Bundle in your Plan then Text UK to UK is Free, Charged for if you go outside your Bundle Allowance or Text Internationally.
If You have a Mobile Data Bundle included in you plan then WhatsApp is Free to send massages to anyone in the World, if you dont have a Data Bundle its still technically cheaper to send a WhatsApp Message over Texting, if your Using WiFi then again WhatsApp is Free to use.
