It's really hard to come to a conclusive answer. Like I said, I use both and... well, here are some more concrete stories:
At 300% TP and near-max HP, I can use Spirits Within for a *guaranteed* 550-580 damage hit. With RK Belt, it takes me 3-3.5 fights to build 300% TP. With Life belt, it takes me 2.5 fights on average (middle of 3rd fight). Also, in some PTs I am included in the renkeis (when I'm with a DRK, we like to Vorpal Scythe -> Seraph Strike for Distortion), and it's good to know that I won't hold the renkei back. Sometimes I even get 100% TP before the other melees.
On the other hand, the DEF and stat bonuses of RK belt are hard to deny. Once I get gluttony sword, I will try RK belt again to see if the +7 +2 VIT will make much of a difference.
Keep in mind that, although most high level PLD I've seen use RK belt, I've seen several that use Life Belt as well.
It's really hard to decide. They're both nearly the same price on my server (RK belt 90k, Life Belt 100k I think... yeah midgardsormr's economy is messed up). I'm thinking at first Life Belt is better, but at later levels when VIT makes more of a difference compared to DEF (and we use WS like Vorpal Blade that give more TP back), I might switch to RK belt again. The experimenting continues
