The M3 is skinnable if you feel the itch. You can make it look nice. However, I do not care for the menus myself, they just add a layer of annoyance in my opinion. The M3 also has a splash screen I do not care for, just another thing to wait through.
The SuperCard is straight to the point, which I like. Power on, bang, list of files. So simple, no nagivating through menus to get what you want.
I bought both and ended up keeping the SuperCard and selling the M3.
Both can run moonshell. Which you can also skin. With the e21 firmware on the M3 (not sure when it was actually added), it even had its own version of moonshell under the 'expand' menu option if I remember right.
I have not had any personal experience with any other carts.
But.. the one I like the best from the pictures I have seen is the G6 Lite.. both the cart itself and the menu look great. I love the icons and if it is really touchable, that is just one more plus for me.
Hopefully some in-depth reviews of the G6 Lite will be rolling out soon. As well as more places to purchase it from.
-Zach