The Unlimited Data Bundle allows you to connect to the internet direct from your phone.
The Unlimited WiFi allows you to connect to wireless hotspots around the country, around 9000 I believe, called something like the cloud agreement or something. Some of the places include coffee shops etc
Unlimited WiFi is only worth it if you have WiFi on your phone, even then, I'd say Unlimited Data is the better option as the data signal is more around than these WiFis.
If you have WiFi on your phone but not the WiFi Bundle, you can still connect to WiFi spots like your route, mates router, some libraries, anywhere with a free connection.
Edit: More info:
http://www.o2.co.uk/mobilestariffs/tariffs/paymonthly/boltons/unlimitedwifi