There is a difference. I've noticed it....MOSTLY when cruising around - D is more aggressive at shifting to a lower gear at a given speed - for example if you're doing 30MPH, it'll drop into 6th, where in manual mode it won't let you go above 5th gear.

That's an example not based on actual data - but relevant as I've tested it. I just don't remember the numbers...

I was always told to use the breaks instead of the engine to slow down. Breaks are cheap to replace compared to the engine

I would says Engine braking causes wear on the gearbox not the engine.

this is the reason why i got my z with stick* lol and im pretty happy with that

so after reading all these posts, it seems like it really doesn't matter which way u do it. Just personal preference i guess. I'll mix it up i guess since i like downshifting sometimes to hear the engine blip and other times i can just let it downshift it self.