it depends on what type of quilts you make which would be the better choices for you- if you make nickle/charm quilts (quilts made with 5" squares) then charms would be good for you to buy---but if you tend to make quilts that you cut larger peices they would not work for you-
if you make strip quilts jelly rolls or honey buns may be right up your alley- but if you tend to make big block quilts they are not going to be very useful-
before spending money on pre-cuts evaluate what you personally find you use the most of-
and remember= they generally are not going to save you money- you pay more (yard for yard) for precuts because you are paying for the convienience of them being pre-cut. if is a good way to get a little bit of a bunch of different fabrics in a line- but they certainly limit what you can do with them.
fat quarters probably offer the most flexability- as long as you don't need any strips longer than 20-22"