I suspect that a UK 'pay and go' dongle is going to cost you SERIOUS money when going on line in Austria. My pay monthly with Vodafone would cost £5 per MB abroad, and pay as you go is not an option abroad with them. As for the rest I'm pretty sure they're much the same. I suspect that a hotspot may be a much better idea, and a lot cheaper in the long run.
John Barden