So out of T Mobile and Vodafone, which one has the best 'unlimited' data???

My iphone 3G o2 sim is used for all sort of things in a windows mobile including hooking up to a laptop and I have never ever had a warning. So it may be better for me to buy an unlocked N900 and use this sim instead.

What do you think?

I should add that I use an o2 open code so pay about £23 a month for 600 mins 500 texts and then the unlimited data.