Thats what I think, If your brother has a SR20DET powered S13, then you know what that car is like, the Z will be a little better as it is lighter Im sure, also if you had your 7MGTE Z running and driving, then you know what that is like.
Both as you would figure already will make good power, you already know what the Toyota motor is capable of, though as owning a few of them in my time as well, they dont handle for poo as you said, and our a bit heavy, now in a Z, I could see this motor making crazy HP, not to mention the torque on the 7MGTE is VERY nice.
I can alsow see where you are going with the SR20DET as well, good power, lighter weight, and not to much power difference out of the box, plus, with it set back you can have a better weight distribution.
I would think that since you have already done the 7MGTE to begin with why not try something new? I think that the SR would be a good choice for an "overall" fun car, for the street, strip and track.
Withstanding one thing, how much power do you want out if it. Ive seen fast Honda's that were tuned by real wrench heads, doing 11's at the track and driving home. So its not said that you cannot have a 4 cylinder that is fast and still very streetable.
My opinion only, Id go for teh SR motor.