Well, it isn't an 86 vs 911 comparison, but I (as a long time lurker) did want to register to give this hopefully relevant opinion.
I've been eyeballing a new BRZ for some time now, and if you asked me a couple months ago what my first choice for a new car would be, I would have said a BRZ, no question. However, a couple weeks ago, I made the mistake of test driving a 2006 Cayman S that was (with everything factored in) about $4-5k more than the BRZ limited (the one I was looking at). Honestly, there's no comparison. Will the Porsche be more expensive to maintain? Absolutely. Is the insurance a bit more expensive than it would be for a BRZ? Yes. Is it worth it? In my opinion, it's worth every penny. As good as the BRZ is, a used Porsche (at least the Cayman S that I ended up buying) is better in almost every way. It has more power without having to wind the engine up near redline (and it still has the instant responsiveness of a naturally aspirated engine), the steering is even more solid feeling and communicative, and the noise it makes at around 5000RPM is just glorious. It's confidence inspiring and fun the way the BRZ is, but the limits are much higher in every respect. The one downside is that the shifter isn't quite as precise as it is in the BRZ, but I can live with that given how much better the steering, brakes, and engine are (and it's a better looking car, in my opinion). Also, as pointed out earlier in the thread, a used Porsche is much farther down the depreciation curve, so chances are you can sell it in a few years for much closer to the purchase price than you could with a BRZ (though this is somewhat offset by the higher cost of ownership).
Now, I will admit that part of the reason I may have been a bit disappointed with the BRZ (especially the power) is that I live in Colorado, and the altitude here really handicaps naturally aspirated engines. The Porsche is probably down on power too, but it started with a bunch more, so even with the altitude penalty, it still feels more than adequate. That having been said, when I was looking at getting a BRZ, I was already planning on an intake, exhaust, ECU, etc to bump the power a bit. With the Porsche, the only thing I've even looked at is a short shift kit, and I don't know if I'll even bother with that.
Honestly, I think your best bet is to go test drive a Porsche. See if you fall hopelessly in love with it (like I did with mine), or if it really doesn't seem much better to you than a BRZ/FRS (which are still great cars, but I personally didn't ever come away from a test drive quite as stunned/amazed as I did with the Cayman).
Oh, and here's a picture of my new wonderful car:
https://www.dropbox.com/s/9okbzjem7k...2018.44.38.jpg