In terms of wear, on a typical torque converter equipped gearbox it’s moot I would say.
Going in and out of neutral will operate the clutches or brake bands but they don’t slip on operation like a manual clutch and they aren’t loaded at that stage, so wear will be immaterial.
On the other hand, leaving it in D will continue to churn the fluid and generate heat, the great enemy of gearboxes - but no much heat, and not for very long. Best avoided perhaps after a long climb or tow, otherwise not a worry.
I do both in my ‘other’ car, a Mitsubishi Outlander with a fluid torque converter. If the brakes are hot, not often the case with me, I don’t like clamping the discs as they cool (some people think this causes warping) so I will use N and the hand brake.
Safety is a separate consideration. The ‘standard’ way to operate an auto seems to be to leave it in D with foot on brake, and I can’t see this changing. Sitting with a car in gear there is always the possibility of inadvertent movement, so I would never leave the gearbox in D with the hand brake on, only the foot-brake. I will use N and the hand brake if I expect to be stopped for more than a minute or so, OR if I have someone queuing behind me at night - a trio of lit up brake lights at short range is no good for anybody’s night vision, whether they are ‘dazzled’ or not.
I looked in Roadcraft to see what it says - nothing on this D/N point that I can find.
A note on DSG - I am not a fan. We have one, on the boss’s Skoda Roomster. The double clutch gearbox was invented by Porsche for racing, as it can make almost instantaneous sequential changes, not specifically as an automatic. Because it is more efficient than a torque converter (though not in my opinion always more effective) the concept has been developed to mimic the functions in use of a traditional TC auto. It’s a fudge, especially in the simulation of creep, which it achieves by slipping its clutch. It’s a pain in the neck for slow, close manoeuvring because if the brake is touched it backs off the clutch instead of maintaining the creep, there is then a pause while it finds the biting point again.
The handbook for the Roomster cautions against “holding the car on the accelerator” and specifies foot firmly on brake when stopped. The foot-brake when stationary fully disengages the clutch. It’s quite noticeable than the hand brake (manual on the Roomster) does not immediately disengage the clutch fully when stopped if the car is left in D. It remains at the biting point for some time, creating wear and heat, until the programming backs the clutch off.
Clearly the programming has been continually tweaked to improve the illusion whilst at the same time limiting wear, but it often has me thinking “what’s it doing now?” When combined with an electro-mechanical computer-controlled hand brake as in my daughter’s Audi A6 it’s even more unsettling, although I have no doubt that a regular driver will inure to it.
On the other hand, I nearly always know what the MX-5 is doing, because I am doing it (glossing over the DBW throttle, ABS, EBD, DSC and TCS!)