I am aspiring to build my dream home with a small cosy tower. Its just a single floor house with a small tower (rook shaped) and 3 bedrooms. I have been looking into forum and one thing that confuses me is about TMT bars, the Fe500 and Fe415. I want to build something crazy ivy or garden in my roof top. So which will be better. The tower needn't be too tall.

PS-I am not a thrifty spender and this house is my dream. I want the tower to function as a small library and room. I am from Kerala and eventhough there are no earthquakes here, I read that Kerala is also in an earthquake prone region and so any suggestions will be great.
Thank you for your help in advance.