Is there a reason? I like the idea of playing it on my SNES, but does the Advance remake make it better? Thought I heard something about the remakes of FF6 removing some famous lines.
SNES vs. GBA is completely an opinion.
Additions: The GBA adds some new espers and a new "end game" dungeon, so if you want the most content definitely go this route. If you only want to play through the game once, definitely play the GBA version since it has more stuff.
Bugs: GBA fixes a lot of weird bugs. The most famous is the evade glitch, where the evade statistic does nothing, so mblock controls your evasion against absolutely everything. You could get a few characters up to 128 mblock (gear dependent), which made them immune to everything in the game besides "sure hit" attacks that you couldn't evade. This made some of the characters really powerful. In the GBA version, evade and mblock are split correctly.
Dialogue: This is where it comes down to opinion. Ted Woolsey, the SNES translator, did not do a perfectly 100% verbatim translation of the Japanese. He instead injected a bit of his own flavor which, in my opinion, improved the game for the better. The GBA translation is a lot more "strict" but loses a lot of the humor and whimsey that Woolsey added, in exchange for some lame puns like "weapons of Magitek destruction." I definitely think the SNES translation is superior to GBA's, but general opinion seems to be split pretty 50/50. But, this is still sorta minor.
If you plan on playing through the game more than once, I'd play the SNES version and then the Advance version. If you're only going to play it once, definitely play the Advance version. Despite the slightly worse translation (again, my opinion), it's wholly a better game. It's still the best Final Fantasy title no matter which system you play it for.