All mobile phone operators do dongle deals, which one is best is difficult to say,at the moment i use two a virgin existing customer deal, at just over £10 a month for 3gb of use,this lasts a month easily with loads of surfing time and some downloads,it would not be of any use for video streaming too slow and not enough allowance.
The other is a vodafone payg deal which is no longer available on it i get 3gb for £15 but only top it up for holidays as that gives me a second choice on coverage and Nancy can update her laptop.
They all seem to have three levels of connection speed depending on the signal that's available,different names are given to these but the vodafone one calls them 3g+,3g,and gprs the two 3g ones are good for normal surfing and uploading pictures,the gprs one is the old mobile phone network so struggles,after saying that the holiday diary thread on this forum was done on gprs from a field in Norfolk it just takes longer.
I have avoided technical details as i dont understand them and have just reported my experiences with dongle use,hopefully it will make sense to you.
mike