Hi Simon,As WC has said it wholly depends what you want it for.
If its targeted at use for keeping in touch with people and info when caravanning then the simplest, lightest and cheapest that you can ergonomically use is what I would recommend. If graphic intensive gaming is what it must do then you need much more computing power.
But if its communicating then get something cheap and make sure you keep it uncluttered of programs you don't need so it starts and operates without much delay. That way you can get the task done and enjoy your caravanning.
If you do not spend much then obsolescences is not an issue.
The other difference I notice between the "30-day expiry" dongles is that they allow 3Gb data forPeter
You are right about the Vodafone website not showing the everlasting dongle, but Argos are still selling them. See:
http://www.argos.co.uk/static/Product/partNumber/5490464/Trail/searchtext>MOBILE BROADBAND.htm
Other shops may have them as well.
Regards