I wouldn't call myself an expert but I've been doing it for about ten years now. I will say A.) If you're going for quality/native fish, flies always will take the cake, but for numbers of 9-12" stockers a spinner will usually outperform because they are so dumb and think anything that moves is food and B.) The water makes a huge difference. There are rivers that I have only caught a handful of trout on spinners, and I always have success on flies, but there are also places where spinners outperform flies nearly consistently. I guess you can target the same fish with streamers but that never can get a reaction strike like a spinner can.
I mostly fly fish because it's fun, especially dry fly takes. I always bring both a fly rod and a spinning rod with me