the difference between charms/layer cakes/and yardage is variety-
you pay for the pre-cuts differently- but whey you get a charm pack or a layer cake you get more variety than if you bought 1 yard of fabric- so you may pay more for that- it is up to you what kind of quilting do you do? if you tend to make small blocks- which you can get out of 5" pieces of fabric- or you do mostly scrappy quilts- charm packs my be just the right thing to buy-
if you need a bit more the layer cakes---and a layer cake cut in half twice gives you (4) charm packs
but if you do not do scrap quilts- if you make larger blocks- you might be better off picking up a yard here and there-
a stash will grow all on it's own as you make quilts- you really do not have to specifically shop to build a stash
decide on a quilt you want to make- go shopping for that quilt- buy 1/2 a yard extra of each fabric you need- that gives you a bit extra for the just in case--(like a wrong cut)
when the project is done---look at all you have left over for your stash!
fabric will multiply like bunnies- if allowed to do so without ever buying a pre-cut pack.
but if you are all about scrappy- and convienience===then which ever pre-cut catches your eye- should be the decider-
you will drive yourself crazy trying to figure out (yardage- conversions) they dont hold up- because you are paying for the variety--and for the fact someone had to cut those fabrics for you.