Hi how exciting. First have you tried either machine? I would have a go on both. Take some cotton fabric and a small quilt sandwich with you to try on. Look at the stitches when you have sewn them - which do you prefer? Which machine runs smoothly? Do you want addition al feet and stitches? If not sure how much will the additional feet cost. You will find that you only use a few feet rather than every one they give you.
As to metal v plastic. probably know which I would go for - but not sure its the best.have you read reviews online? Buying a new machine is pretty scary but good luck