While I agree that Duncan is more of a center at this stage, he's almost universally listed as a power forward on "best of" lists.
I'd prefer Duncan. I think they're equal defenders and basically equal rebounders (although KG's numbers have been down in Boston). However, I think Duncan can score in more ways. Also, I think Duncan has always had that extra something to get his team over the hump; four rings counts for a lot in my book.
Also, I think KG is closer to end (although I hope neither one retires any time soon), only because he's got a lot more wear and tear on his body. Even though the two are the same age, KG has been playing for two extra seasons. Garnett has played an extra 6500+ minutes, which is a fairly significant amount. In recent seasons, Duncan has been more durable (15 games missed in the past four seasons, as compared to 48 for KG, including 6+ in each of those seasons), and his stats haven't declined as much from their peak.