Average, the way the book says to do it.
That is, at odd levels, they gain the odd 'half' of average (5 on a d8 or d10, 3 on a d4). At even levels, they gain the even 'half' (4 on a d8, 6 on a d10, 2 on a d4).
I much prefer it to rolling. The way I see it, HP is such an abstraction that the fact that two characters of the same level, HD, and Con mod have the same HP isn't any stranger or more 'artificial' than them having the same BAB or saves, or whatever. If you're going to roll something as abstract as HP (and therfore make it uneven between characters), you might as well roll XP randomly.