Go with 9 mil.
My first box was 7 mil. They work fine for a quick job, such as an oil change. But for more lengthy jobs, I'll end up having to put on new gloves once or twice before the job is done. And it becomes a real annoyance to have to stop and change a glove. Especially when you realize a glove is torn and your hand is getting dirty. It kind of defeats the purpose.
Now I use 9 mil, and they easily last throughout a job. In fact some times I'll save a pair to use for another job, because they are in such good shape.
Find a brand with a long cuff also. It sure helps keep you cleaner.