They're not necessarily exclusive of each other. But the "reason" you might want to use one is sometimes contrary to why you might want to use the other one. Much will depend on the specific site you are hunting. Fast ON is typically used in sites with an abundance of targets as it accelerates the signal processing "between" targets. The upside is that you reduce the lag (reset time) between two adjacent targets. The downside is that the TID's will not be as accurate.
Deep ON is used to enhance the signal of deeply buried targets and is recommended (by Minelab) to be used in "uncluttered" areas. Due to the extra filtering of Deep ON, you will have a slight delay in both audio resopnse and target identification information. With that said, if you look at the factory preset Relic Search mode of your CTX 3030, you'll see that it has both Fast ON and Deep ON, in combination with Smooth audio response. I suppose if you don't mind having your TID numbers skewed a bit, and you don't mind a slight delay in the TID information being displayed, having both on would allow the CTX 3030 to reset between targets more quickly, and it would enhance the signal of the deeply buried targets. And by having the response set to smooth, you may not notice the delay in audio response. This combination would provide an almost continuous audio response, filtered to reduce the difference between the "high and low" tones. HH Randy