For giggles I went to my local Porsche dealer to look at a Cayman T. I saw that they had one for like $8XXXX. Matter of fact,
here it is, still unsold. Anyway, my M2C is a DD, for my needs, its perfect. It can seat 4, I can put the seats down and actually have a trunk, it's fast in a straight line and corners very well. And most importantly, the car is just an excellent driver, it is 100% a driver's car. I know that the 718 is going to have some of those characteristics, but not ones that make it a good DD in my opinion. I think the M3/4 is in the same boat but to a bigger extent. In the M3, you can have even more space.
I think this is a long winded way of saying, if you are DDing a car and need space or want something that you can hoon around in but at the same time take a cruise to a dinner date, the G8X makes more sense.
You know what I would honestly recommend? Take both out. You have time before the X Drive arrives. See what sticks in your mind. If lets say in a week or a month, you can't get the 718 out of your head, stop waiting and buy that. But if you feel like the G8X is 80% of the fun but has the utility that you need, it may be worth the wait.