The Orange deal is a contract deal. PAYG is an up front payment for the USB thingy and then same as a phone you pay for what you use. Prices are coming down I notice as they are becoming more popular. Don't jump in too soon would be my motto. Since I started looking some months ago PAYG has dropped by about 25% on some companies.
I agree hound, 3 are offering me £15 a month for a 15Gb download limit and a free dongle, close to what I want but not quite there yet. It will end up being a hell of a lot cheaper than getting a landline set up as bastard BT want to charge me £125 to send an engineer round to test the line is still working as it hasnt been used in a while, and thats before the connection fee. Customer service bint on the phone didnt appreciate me telling her that I used to work for BT and that they could do the same test by sending a signal to the phone I had plugged in for free.