I still believe KG was a comparable talent to Tim Duncan for about a decade. While Duncan was winning championships, KG just didn't have a supporting cast. It was basically proven in 2008 when post-prime KG got a couple sidekicks and won a title on his first attempt.
But yeah, Dirk has been an elite offensive player for a while. Dominant defensive bigs seem to be the most important factor to building a champion on this league, though.
If you surrounded Dirk and KG with comparable talent for 15 years, I think KG wins you more titles. Plus, KG was pretty darn effective for 17 years... Dirk has been pretty darn effective for 15 going on 16... DIrk's window might end up being bigger, but KG's dominant 15 year run helps him take home more gold.