Very personal musings follow, YMMV:
I have been pondering this question as well, ever since the P85D unveiling. My goal for Tesla was to buy into it fairly early (early for my market anyway, early obviously means different things in different parts of the globe) and quickly, and the P85 did that trick, but for the longer-term to get a four-wheel drive model once available.
Having driven plenty of German four-wheel driven premium sedans/saloons in my time, and less SUVs, I actually might well be in the market for a P85D judging by that, considering how sedan-like a hatchback it is. I certainly enjoy my P85 a lot, easily one of the top three or top two cars I've ever owned, perhaps the best in terms of the glee-factor. Its basic design is genius, how it frees space and goes for simplicity in structure compared to an average ICE or even ICE-like designed EV (Roadster being in the latter group). And it is a really pretty vehicle.
Yet, I don't want a P85D. I have given it some serious thought since obviously I could convert my Model X Signature reservation into a P85D order, if I wanted to. But buying another premium sedan or sedan-like hatchback wasn't all that interesting in the first place when I got the P85, I got the Model S because I felt compelled by its EVness and a lot of the design decisions that went into it (see the link in my signature), not so much by the form-factor of the car itself. Even if P85D had been available a year ago, going for that for the longer-term wouldn't exactly be right for me. Maybe I've grown tired of sedans personally or maybe it is because Model S still misses some things.
Model X is, as a form-factor or a vehicle configuration, much more interesting to me at this time - and leaving the experience of AWD EVing to that is an added bonus. I'll let the Model S be the performance RWD hatchback/sedan/saloon, and wait for the Model X for the SUV AWD thing. It also helps that Model X probably is the premium interior experience compared to the somewhat spartan Model S. I get it that Model X is basically an improved Model S, but overall that that a good thing. It improves things for me, I expect, to such a level to be really, really excited about.
So, I'll let each car have their own personality. I will probably select a different colors inside and outside for the Model X, too, for a wholly new experience. Model S is a bit of a beautiful detour and a pioneer of a car for me, Model X is the one I feel really strongly about as a vehicle, personally, not just as an idea.
I expect Model X to mature the idea of Model S to a wholly exciting car and that's what it's about for me. Who am I kidding, it's all about the falcon wings.