Cid dies regardless. If I remember, if you keep him alive, he's the one that commits suicide. That's pretty vauge though since I always fail at saving him and watch the scene where Celes fails at suicide.
...No, it doesn't matter if you keep him alive. He's a usless sack of bones pretty much.
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"Melding with the Planet...
...I will cease to exist as I am now, only to be reborn as a 'God' to rule over every soul."