in most cases curaja, however in ffxii it tragets all (curaga targets one), so it may not be as effective
Technically, curaga is more powerful. Curaja was previously known as Full-cure, and as such, will fully heal a party memebers HP. However, since Curaga is based on several mathematical factors which vary from game to game. Because of this, Curaga could have a "healing value" which exceeds the maximum possible HP. This would not affect gameplay in anyway, of course.
To sum up, If you have the MP for it, Curaja is better.