The only things I'd say the SNES has over the GBA is the resolution of the images
The graphics are the same resolution. Only the output resolution is higher, which allows for more content to be shown on the edges of the screen.
The GBA screen is 240x160 while the SNES can be 512 x 448.
It can be, but FFVI does not use the SNES' high-res mode; it runs at 256x224, only marginally bigger horizontally than the GBA (though considerably bigger vertically). I doubt it can be made to run at 512x448 without considerable reworking to the game's graphics engine.
Also, if we wanna get really pedantic, the SNES runs at 512x480 or 256x240, but the difference is pretty much black bars at the top and bottom, an area of screen that AFAIK no game ever rendered to.
Retranslating would mean expanding the 24-meg game into a 32-meg one. That's not that hard a conversion (I don't think), but no one's taken the time to do it.
It's been done. RPGOne's translation did just that.
I know resolution, But i think the background images were the same size, Just SNES expanded the images (could be wrong)
http://images2.wikia.nocookie.net/__cb20091016222204/finalfantasy/images/4/48/FFVI_Gba_vs_Snes.png
As seen here the image looks the same, just one is brighter to compensate for the dim screen on the GBA
The background images are indeed the same size, but the brightening frankly screws up the tone of the game when played on anything but an original, non-backlit GBA. Areas that are meant to be dark and moody become far more bright and pleasant.
Many GBA games were developed with the intent to work correctly on both a GBA and a Game Boy Player (or if it was late enough, even a DS). This is apparent in games like
A Link to the Past, which offered a brightness setting, so that GBA users could still see, but GB Player users weren't stuck with washed out colors. In spite of being pretty much the last major GBA release, I'm pretty sure FF6 Advance doesn't have a setting like this. IMO, this is in and of itself a strong argument in favor of the SNES version.