As to cotton vs. poly batting on a practice piece...... it depends.
If you are just practicing for the sake of getting to know the feel of the machine, then no, it doesn't really matter. Once you master those loops (and it won't be long, trust me!), you will be paying more attention to the quality of your stitches, and the tension of your threads. That's when a poly batting can either help you --- or hurt you. Confused yet?
A poly batting may hide the quality of your stitches, and that can be a good thing, BUT...... if and when you do switch over to cotton batting, you may see a lot of change in the way your stitches actually look. If you don't have a stitch regulator, for example, cotton batting will make it easier to see the length of each stitch. If that's something you are trying to master (even stitch length), you may want to use cotton batting and contrasting thread.
If you just want to be able to finish pretty quilts, and actually USE them (smile), then poly should be fine. Just keep an eye on your tension settings when switching back and forth between the two, because you could have some surprises on the back side!