It's not a question of 'better'. Without going all technical on you, the
speed rating of RAM tells you the maximum speed at which the module will
read or write information. That rating is a theoretical maximum. It doesn't
mean that 533MHz memory is 'better' than 400MHz memory. You would be hard
pressed to notice any difference. Moreover, the speed of a RAM module is
only one of very many characteristics that determine the maximum theoretical
speed at which a computer will operate.
So you shouldn't obsess over that tiny detail. If the price difference of
the faster memory is small, may as well go for it. But don't get all bent
out of shape over it - it's really no big deal.