Both are, in my opinion, quite different games, so I'm not sure about the comparison.
Amazingly, despite being one of the longest running game series, I have never actually played a Final Fantasy game. I have seen trailers, gameplay footages and played a 10 minute demo of one of the PS2 versions, but never any of the full games. I'm not sure why but they never really appealed much to me. As a result I can't comment much on the game.
I bought a copy of Dynasty Warriors 5 for the PS2 a few years back after enjoying the demo version, and I instantly loved it. For me, the different storylines, scenarios and moves for each character took away the tediousness of the constant killing. Combine that with all of the unlockables and the multiplayer mode and you have one very addictive game!
I have yet to play the PS3 version, but I can't wait to!