It all depends on how you intend to use them.
I suggest an 18-weight set (case or not: I got the case to protect the magnets and to protect other things FROM the magnets, as well as to store them).
In order to add up to 4.5# on TWO dummbells, you would need the 18-weight set. And being able to add 1/2# at a time, until you get 1/2# away from the next weight increment, is very useful, IMO.t a
9 pace weights only add up to 4.5# total, so only 2.25# per dumbell, which would still leave a 2.75# gap between one increment and the next (which doesn't seem like much, but if you can go up in smaller increments---microload---it's easier to be able to lift more without your body noticing).