I bought FFIII the day it came out and couldn't put it down until I was done, I really enjoyed it. But, if you've never played FFVI, then there's no contest. Don't be drawn in by the 3D graphics on the DS, the 16-bit old-school still has just as much charm!
That, and it wouldn't matter how good or bad it looked, because it's the story and characters which make most people consider it to be one of the (if not THE) best RPGs ever made, myself included.