Mobile broadband deals? Who is best? o2 or Vodafone?

I am looking at getting a pay as you get mobile broadband dongle for my laptop as I am photographer and software developer and I would like to be able to get online anywhere. I don't want a contract as I already have one with my iPhone and i don't think i would use it enough to warrant the extra monthly fee. The 2 deals I am looking at is Vodafone and o2. Vodafone seem better as the data can be used at anytime and i only pay when I need more, the downside it that it currently doesn't work with mac's at this moment in time. On the other hand o2 does work in OS X but the prices arn't good. £2 for 500mb, seems fine but it only lasts a day. Other options are £7.50 for 7 days and £15 for 30 days.

Anybody used any of these products? I am thinking about the o2 dongle but I really like the pricing plan with Vodafone.... Hmmm what to do