it depends on the use of the quilt when it's done- and how you plan to quilt it- so make the quilt first-then worry about the batting
some batts need to be quilted every 2" (which is quite dense quilting) others allow up to 10" apart between quilting lines- which allows large panto's and larger designs- it is important to buy the batting that is right for your quilt---so you need to decide those things before purchasing it. read the packages-
poly batts hold up well to lots of abuse- good for kids quilt- cotton batts are good- wool batts are wonderful but expensive.
batts come in many different (lofts) do you want a fluffy quilt? or a flat quilt?
read the packaging on all the batts you come across- and experiment- prehaps pick up 2 or 3 different batts in craft sizes to use for your quilting practice pieces so you can see what they are like to work with...