I'm going to take a semi-educated guess, being a fan of astrology myself (though definitely no expert!): looks like predictive astrology takes a look at how likely an event will be based on the positions of the planets within the zodiac, and how the planets relate to each other (through squares, trines, etc.). What I don't know is if predictive astrology allows for the intuitive leaps that Tarot does, so I'll leave that to those who know it intimately...
To me, it's not which system IS better - the question is, which system is better for me, personally. For me, it's Tarot.
Sure, I subscribe to a free horoscope, but I couldn't tell you if it's likely something will happen to me based on Planet A's opposition to Planet B and why. I could tell you better with Tarot; that's the "language" I speak.
I'm gonna go out on a limb for this next sentence: I do believe that both systems can predict specific events. Ancient astrologers had been doing it for kings and queens for centuries (by how much success? Couldn't tell you). Italian cartomancy (there was a thread around here about that a while ago)... VERY specific, which it seems to me here in the west, we don't even try to do...
Well, even if I'm wrong, all I really want to say is that it all lies in belief. Believing in your divination system gives that system power, and imho, I see that the more power people puts in themselves and their system, the better that system seems to work (for them).