a jelly roll is going to give you a selection of 2 1/2" strips---
a layer cake is 10" squares....
so if you are making a strip quilt- jelly rolls are great- or any pattern that the pieces are not larger than 2 1/2" wide---
the layer cake gives you more (surface) to make larger cuts...area wise- they are about the same ---(4) 2 1/2"x10" strips from 1 layer cake vs. (1) 2 1/2"x 40"...so it only depends on how long of pieces you need---and width---
can you get your pieces from the square- or will the jelly roll work?
there are some pretty good books on jelly roll quilts-
i think you should choose a pattern before purchasing--or if you just want stash fabric for future...collect both...:)