Next vehicle narrowed down.....

ZCanadian

So, what's the purpose for the car?

Sounds like you might not be clear on that yourself just yet.

The Tacoma you bought has a fair bit of luxury, and is a pretty economical and practical vehicle (unless you deal with tight parking spots or only bought the access cab and want to take rear passengers who aren't double-amputees).
So do you want/need another luxury car?

If yes, then the Lexus all the way. I fear it might disappoint on the performance scale (seems to me, reviewers are split on a bunch of the newer Lexi in that regard, but I've also learned not to give much credit to their reports anymore).

The Mustang is pretty cool these days, and in GT350 trim it is both very sporty and still somewhat usable for travel beyond getting back and forth to a race track. But I don't see it as a luxury GT cruiser the same way as the RC-F is.

Since your are considering replacing not just a Z, but a NISMO version, I'm thinking the Mustang would be more up your ally. But that's making a lot of assumptions on my part.

I don't think you can make a wrong choice here, except if you want something out of a car that it isn't intended to provide. Certainly, a year older Lexus will be a better value than a newer Mustang. And the Toyota will likely be more reliable and hold its value better in the long run (if you plan to keep it for a while).

But, if we are talking next year, there could be some very nice third alternatives that come up in the mean time. So go ahead and dream, but keep your mind open to that as well.