Don't forget the PSN version. That one is likely more readily available to people than the SNES or GBA releases, and doesn't have the diminished sound quality of the the GBA game. And being digital cuts down on loading times from the original PS1 release.
This is a good point! Though even digitally, the load times are still agonising compared to the SNES version. Scene transitions into and out of battle scenes take far longer than they should, and it all adds up in a game with so many battles.
The SNES version is actually available as a download via the eShop on the original Wii (the Wii U can also access this) so that’s always an option. Though, if you have a Vita the load times of the PS1 version are a small price to pay for portability.
You mentioned that you'd be picking up the mobile port of FF6 despite your reservations. That was a few months ago, what's your verdict? As someone who hasn't played FF6 (I know, the shame), is it bad enough that I should avoid having it be my first real exposure to the game?
I did play and finish the iOS port just recently.
While it’s not outright horrible, there’s no way I can in good conscience recommend it as your first exposure to the game. The SNES version is still my preferred way to play through it, though the GBA port is alright, though the sound and colour quality is inferior to the SNES original.