In terms of the world ranking, IISc is way behind from NUS. In QS world ranking IISc is ranked at 185th position whereas NUS is at 11th position.

NUS covers a wide variety of courses like Business, Arts and Social Sciences, Computing, Dentistry, Design and environment, Duke-NUS Medical School, Engineering, Integrative Sciences and Engineering, Law, Music, Public Health, Public Policy, etc. Whereas in IISC there are two centers- Science and Engineering. Both of them provide undergraduate, postgraduate, and Ph.D. programs. IISC mainly focuses on research work and provides only BS degrees at the bachelor level. NTU also has limited courses at bachelor level like Biology, Chemistry, Physics, etc.

NTU has good professors and has much better infrastructure, facilities, and global exposure. Because of this, the fee is very high in NUS if compared with IISc. The students are expected to deliver rather than understanding the basics. They have to study advanced subjects in the first semester itself at NUT.
IISc is best if you want to stay in India. But if you want global exposure then go for NUS.