The natal. It helps to bear in mind the progressed chart is essentially an encapsulation of planetary influences within the first 90 days of life, intended to symbolize our individual evolution for the next 90 years of life. Give or take.
For this reason, secondary progressions serve much better when examining perfecting aspects or as transits to the natal chart, but I don’t find transits to secondary progressed planets make a great deal of sense (regardless of what Carol Rushman says on CafeAstrology or similar sites).
Bernadette Brady writes in the book Predictive Astrology: The Eagle and the Lark (an underrated author, btw) that secondary progressed planets do not receive the same energetic influence as natal points do from transiting planets. I agree, and this tends to be the general consensus, amongst many astrologers I know.
I think, because the progressed chart is marketed as “a character evolution” we tend to think that at some point it will better represent our truest, most actualized self, so we try to apply the same techniques we find useful with the natal chart, but they lose their effectiveness. I suppose, due to a the fact that all the hours contained within the parentheses of life and death are a little less momentous.
That said, I often find secondary progressions (amongst other timing techniques) even more accurate than transits for reasons I won’t yammer on about. Transits are the most popular form of predictive astrology, but plenty of transits yield unremarkable outcomes because the potential simply didn’t exist natally. There are additional techniques I prefer and other factors to consider when practicing predictive astrology.