Thats a tough one for us to answer - this very much depends on how much you use! The best way to look at it is to consider how much time you spend connected to wi-fi and how much you use your phone when you're out of range of a wi-fi - like commuting to work, driving, walking around, exercising etc
When you're on wi-fi you aren't using the data in your wireless plan at all
Most people who use our website are looking for between 2GB and 3GB of data per line - this is perfectly sufficient if you are an average user of data - running maps on your phone for directions, checking emails, facebook etc. If you stream a lot of music or videos while not connected to wi-fi then you will need more data, maybe upwards of 5GB
Unlimited data plans are good value but only if you use a load of data!
Heres a link to the best plans available with different amounts of data: