Thanks all for your iteresting "facts" – to summarise:
cup and cone = excellent if you remember to religiously service them even when you think they're fine (I'd like to think I'd do this, but probably won't), and once you've accidentally forgotten to service them (for a few years) they could be irreparably damaged
cartridge e.g. hope – cost a few pounds more per year but you can ignore them until something's wrong, and they have a bit more resistance (though that probably wont make a noticeable difference, I remember reading an article by Lennard Zinn about it here)
I have some ritchey road sealed ones, which I'd like to have messed about with, but the freehub body is too soft and the casette has made big dents in it! That's a design fault.