You obviously understand and personally appreciate the advantage of expandable memory. For me this is also a crucial factor. Otherwise, I like what I've heard about the N4 - It's fast!
However, the S3's camera is very good whereas the N4 has received poor reviews in this area. Seriously, the camera on the S3 is great. I'm talking about the actual lens and sensor, not the software. The samsung camera software is very good, but doesn't have the N4's photosphere. However if you root your phone you can install the N4 camera on your samsung and have two camera apps as I myself have done.
The Samsung has more software features if you like that sort of thing. I really like Smart Stay which checks to see if I'm looking at the screen to see whether to time out the screen and save power. This works very well - I hardly ever have the screen going off when I am looking at it.
There's also multi-window view, but I don't use it to be honest.
One thing the S3 does out of the box - USB OTG. Purchase a cheap USB OTG cable and you can use your S3 as a USB Host i.e. you can plug in a usb thumb drive, or a usb keyboard or mouse, etc. just like you can on a computer. It makes your phone like a computer!
The N4 should have the USB OTG capability, but it doesn't out of the box. You would have to root, and flash a modified kernel to obtain this functionality.
Another thing. The S3 has standardized USB Audio Out, which Samsung added to the inadequate Jelly Bean implementation. If you're a HiFi buff you can buy an external DAC/Amp that will plug into your USB OTG cable and you have awesome quality sound for your audiophile headphones or even full size hifi audio system.
As you mention the prospect of carrying a lot of media/audio around, this is a direction you could possibly expand in.
People forget how awesome the S3 really is.
EDIT: as to the 'flimsiness' I never notice anything of the sort in my Incipio Silicrylic case. But I quite liked it 'naked' anyway. I wouldn't let general comments about 'flimsiness' put you off. As john_g said, 'it's simply an intelligent use of modern materials.'