My daughter has her first competition in December. She has a cheap but functional spirit brand gi. Is it worth investing in a new Gi with a bit more weight? I was thinking core black label. I should add she's 9 and currently a white belt but should hopefully get her yellow after the comp.
No for a couple of reasons. She's a white belt, she may get battered in the tournament and decide she doesn't want to continue. Also putting her in a heavier gi might might throw her off a bit from what she's comfortable in. A new gi will do nothing to help her in the tournament.
If you buy an expensive gi, the likelihood is that is will be in equally bad shape after a 'heavy' tournament. More expensive gis are usually better in training than in the competition side as it can cost you a lot of money. Weight in a gi I like; for me the heavier the better, however I would buy a cheaper heavyweight gi for your daughter to try first to make sure she likes it. If she does then you can buy a more expensive one at a later date.