I'd like to see price comparisons between those versions and this version when it comes out. As far as I know the PSX version is pretty bad and the SNES version isn't exactly cheap. And even if you can get your hands on something like the SNES version god help you if you don't have an SDTV anymore.
GBAs are fairly cheap and will probably last longer than your iPhone/Android phone. You can pick up a FFVI cart from Amazon for as low as $25. (Considering Square would probably charge up to $25 for this, it's better to go the cart route. You get something that was actually worked on properly and something physical for your collection)
you can also get FFIII from the Wii VC