In terms of strenght and usefulness, her decision was good. But it's not like she decided like that with that thought in mind.
It's certainly understandable why she did. After all, she loves Kei. And let's not forget that she even helped Tae when she was targeted. That must've been hard.
I don't like her decision to bring back Kei, simply because he was not dead. And wishing her own Kei into existince is creepy and wrong in my opinion.
And when Oku downplayed it, I got disappointed. How can the original be like "Perfect, now we can have both. Good luck buddy". Even in the gantz world, seeing another you should have a different effect. It's not the same as seeing a second Kei (the girl).
But, why not put your questions in the 'gantz manga discussion' thread next time? With the amount of people posting these days, it's better than making an own thread for everything that only get's a few posts.
"For Feanor was made the mightiest in all parts of body and mind, in valour, in endurance, in beauty, in understanding, in skill, in strength and subtlety alike, of all the Children of Illuvatar, and a bright flame was in him"