hi there, i think you might of found the cause to the data ammounts, as the vodafone software monitors the useage of the pc its installed to, so if its installed on 2 pc`s then effectively you would need to add the 2 ammounts together to see the data useage of that stick
that said, if you collate all the data unbilled through the online my account system, that is for the stick, which would provide that total ammount.
as for how long it would last, the average is roughly 30 hours per Gb, however thats purely browsing, so your 3Gb would give 90 - 100 hours a month, which is alot, say 3 hours a day or so, that said use of sites such as youtube, iplayer and vimeo which are media streaming sites do use up alot more bandwidth, and downloads even more, with say a film being 700Mb as an average size!
your right about the £25 option not sounding so good because you have that deal, however they are currently running a deal of 3Gb for £10 for existing customers, not as good as what you have at the moment, but that would be 6Gb for £17.50, also then each user could keep their own stick giving them a) more freedom and b) a clearer indicator to their useage as sticks wouldnt be getting swapped, as a word of caution though that offer is apparently ending soon, so might be worth making enquirys before it goes!
the other offer, which is very good if you have a business account, is £18+vat a month over 18 months and you get Unlimited* data allowance, so you know you`ll only ever have that one flat fee
* while it truly is unlimited, after you reach 5Gb the speeds are throttled so as to avoid network congestion from abuse.
hope that gives ya some answers and some ideas