I wouldn't do either. Most people have bad hair after the keratin treatment wears off.
It masks the damage that the straightener does. It really depends on which one you get, and it's really the luck of the draw.

I think it would be easier to keep you hair up while it grows. I had one done and it messed up my curls a bit. They were much looser until a year after the treatment.

Are you sure your hair is that curly still? When I was 6 I had straight hair, now I don't.
Your hair might be growing in looser now.

You could always try to make heat free curls, it could help the transition, I think.
I think using the treatment might do more damage than good over already relaxed hair.