GIFFGAFF iPhone 5 & Internet?

I'm getting an unlocked iPhone 5 new and im planning on using Giffgaff (most my contacts are giffgaffers).
I'm staying on the standard pricing and not planning on buying a goodybag because it will go to waste. I only text my Giffgaff contacts but text and call other networks and landline occasionally but I do use whatsapp and twitter.
Would standard pricing be better of with me including the 20p megabite allowance?
(PS. What is the 20p megabite allowance on GiffGaff can you explain to me and how I can use/activate it?)

Sorry I sound like a cheeseball

Added (1). How much MB would be suitable for the use of iPhone 5 because all i would really do is use whatsapp and Twitter (Heavy use). However im mostly at home so i can use wifi but when im out thats when i would go airplane mode and not bother with the internet. If i just tweet 5 messages at most or send a couple of whatsapp messages, do you think thatd be worth it for 20p a day? Is 20p a day really worth it and how much would that last me?