Both the universities are well renowned and are known for their quality education and placements. Whereas, the only difference lies in the selection procedure. 

IISERs only accept admissions on the basis of JEE Advanced scores. Thus, giving you a better peer group. But SNU also offers a good BSc/MSc program for Physics. 

Thus, choosing any one of them is definitely a tough task. But given the choice between the two, students tend to go with IISER, due to its selection factor.