you can visit the warm company they tell lots about different batting properties and uses.
alot of it is just personal choice; when i started quilting i tried lots of different batts, each one i would cut a 6" square from, write down the company, fiber content, stitch requirements, cost, packaging, anything i thought i would want to know later if i wanted to buy it again. i also documented any issues i had using it or likes/dislikes. my (book) has about 50 different batting swatches in it now.
there are some people who use one batting...for ever...that is what they like and they see no reason to try anything else.
buying what is on sale is a good way to try different ones but you should pay attention to the things you like and do not like about it, so if you HATE using it you don't buy it again 6 months down the road because you forgot thats the one you hate.
some people hate poly batts and swear they will melt or fall apart or something horrible will happen if you use them. i've used lots of poly batts, i've never had one melt, fall apart, distort, or do anything bad. and they hold up very well for kids quilts that get washed alot. poly wears very well. cotton batts have tons of different weights and properties, some are better for hand quilting than for machine quilting, some are very thin, some quite lofty, all shrink a little bit. (poly's do not shrink)
i absolutely love the Dream Battings! dream wool my favorite, dream poly comes next. the bamboo/silk is fabulous to work with but very expensive.
there is no reason to stress over your batting, try different ones, keep a little journal and as you go along you will decide what your favorites are, and they may be totally different then the gal at the lqs...