Are IISERs better than IITs in terms of research and scope for higher education abroad in basic sciences like physics?

If you wish to pursue Physics or any branch of pure science, you must opt for IISc, IISERs, and IITs in that order only. If you wish to have a career in research, then you should opt for the first two. If you wish to grab a good job, then you should go for an IIT.

IITs are usually made of 40% research and 60% teaching and it is good if you are willing to do a B.Tech and join industry jobs. IISERs are more research-oriented. 

Both IITs and IISERs have nice faculty with good research labs and research papers. It would be better to read the findings of the research papers of the faculty under whom you would be studying, before choosing an institute. If you wish to pursue higher education outside of India, you can do that anyway even if you are not from an IIT or IISER, or IISc. So, do your research and choose according to your convenience.