The GT-5 was the first of the series, followed by GT-3 few years later , and now the GT-6... They are all great , same awsome sound quality you'd expect from a Boss.
But for sure the newer should be more updated somehow, new effects added, etc.
I haven't tried the specific korg model you've mentioned... But I can tell you that if you can afford either Boss GT-3 or GT-6 , get it & you'll never feel sorry.