IISc and IISER both are good. I can be biased being an IISc student but I will prefer IISc over IISERs. IISc is quite established institute and have been renowned for many decades. Its undergraduate programme is new and I think the first batch is passing out this year only. What makes IISc great is its research atmosphere. IISc is primarily a research institute than a college. So you get a nice exposure for research at very early stage. In IISc you can expect a world class research if not a nobel class. Some faculties are extremely good. You can imagine by the fact that despite sitting for placements a lot of students opt for higher education. IISc research culture is quite influential. But Student population in UG is quite slim compared to IISERs. This might have some cons
IISERs are also fast adapting themselves for science education and are doing commendable job. If you want to go abroad for research might be IISER atmosphere can be more helpful because of large student population having same mindset. IISERs faculties are mostly new and more enthusiastic. IISER also has a wonderful research atmosphere.
In short both options are good for undergraduation but given the choice I will go with IISc.
Abhay Tiwari, IISc Bangalore