Duncan was basically a superstar from day 1. His "peak" if you will began the minute he entered the league. Whereas for KG it took him 4-5 seasons to really become a superstar talent. Nothing wrong with that, that's the normal curve for star players but that's part of why Duncan's peak lasted longer. Additionally KG's peak really ended starting his 2nd season in Boston. By that point at age 32 he was only averaging 16/8/2/1/1. Nothing wrong with that, still exceptional numbers, but clearly not the same KG we had come to expect. Overall I'd say KG's peak lasted from the 98-99 season (age 22) until the 07-08 season (age 31). An incredible 10 year stretch.
However, as I noted, Duncan's was longer no doubt. I'd argue Duncan's peak was from 97-98 (age 21, rookie year) until 09-10 (age 33) a 13 year span. On top of that, Duncan's fall post-peak was nowhere near as extreme as KG's. Even in Duncan's final season he was still one of the 3 or 4 most important players on a Spurs team that was a very serious title contender. While KG was nothing more than a bench presence for the T-Wolves. Hell, even in Duncan's age 36 season he averaged 18/10/3 with 2.7 blocks per game! KG's career high was 2.2. And for reference KG in his age 36 season averaged 15/8/2/1/1. Nothing wrong with those numbers but not quite on par with Duncan.
I'll give you this though, KG was a more versatile player. He could defend the perimeter (although you didn't WANT to see him doing it) and he had that sweet 18-20 foot jumper from the top of the key. Duncan never developed a consistent shot from that range, he did his best work with his back to the basket. KG on the other hand had a slightly bigger arsenal to score from. That's why for the purposes of my starting 5 I picked KG, even though I do think Duncan was the better player.
I like both players a lot, so I'm not trying to put anyone down who picks KG. But in my opinion while KG is an all-time great, Duncan is still a tier above him.