Which wadding/batting should I buy?

The question of which batting/wadding to use is quite complex as the choices are many. Here are some of the considerations you might want to think about.

Consider the look you will get, and the ease with which the wadding can be sewn. For example in hand quilting soft, open wool wadding will be much easier to sew than firm 100% cotton. For machine quilting 100% cotton will be easier to use than a high loft polyester wadding, which can be hard to get through the sewing machine, and which doesn't 'stick' the fabric layers together very well.

How appropriate the wadding is for the end use of the piece? Wool wadding would not hold a large wall hanging well and would give a wavy finish as it is too soft and fluffy. However wool wadding would be great for a bed quilt because it is drapes well and is hypoallergenic, breathable and warm. A 100% cotton wadding would give good results for a large wall hanging because it has the stability required.

Who is going to use the quilt? For example polyester wadding is not suitable for toddlers and young children as they cannot regulate their heat, so you would choose a natural fibre. Some sources say babies should not sleep under quilts (they should not sleep under duvets because they can get too hot and are not able to throw the covers off). My baby sons slept in a cold bedroom under a cotton sheet, cellular wool blanket and a cotton quilt which had an old, much washed, woollen blanket for the wadding. Common sense and some careful research are your guide here.

Cost is another consideration. Large pieces of wadding can be expensive and some fibres are cheaper than others.

Then you get into environmental and ecological considerations. Do you choose organic cotton (cotton growing uses more water and pesticides/insecticides and fertilisers than just about any other crop) or wadding made from recycled plastic bottles?

Experience and experimentation will help you make the right choices. For a beginner, machine sewn, quilt I suggest either 100% cotton or 80% cotton/20% polyester. I soak both these types of wadding prior to use (warm water, no detergent). They are then spun and air dried. This shrinks, softens and makes the surface a little less smooth so the fabrics stick to it better.

Hope that gives you some insight of where to begin your research and purchases. :)