There are arguments both ways with little good evidence to support the strong opinions. In such cases, it is often best to see what your vet thinks and go with his advice.
I would go ahead now, heading off having to go through her season. An accidental breeding and unwanted litter is not the worst thing an inexperienced owner can have happen. Perhaps females are better off waiting to be spayed, but having a litter at 9 months isn't good for them either. Then there are infections and even getting loose and getting killed.