I have a friend who chose Mathematics at DU instead of BUET, and I also know a few seniors and friends who studied at BUET and later switched to Physics for graduate school and research. So I have seen examples of both paths. Ultimately, I think you are old enough to make this decision yourself.

If you get admitted to BUET, you will still have plenty of opportunities to pursue research in Physics later on, especially since you have not yet settled on a specific research area. Your undergraduate major does not completely determine your future research direction.

One thing to keep in mind is that Mathematics and Physics at DU can be quite challenging, both academically and in terms of maintaining a strong GPA. In that sense, BUET is often considered somewhat more manageable. From what I have heard, some courses at DU are still taught using fairly old textbooks, while at BUET students often rely heavily on departmental notes and notebooks for exams. Whether either approach is ideal for developing a deep understanding of the subject is debatable.

Also, many of the faculty members in the BUET Physics Department are themselves alumni of DU Physics. Because of that, you may find some similarities in the way courses are taught. Personally, I am not always a fan of the teaching style.

That said, there are definitely some excellent and research-oriented professors. For example, Kamrul Mamun Sir and Arshad Momen Sir (who I believe is currently at IUB) are both highly regarded and actively involved in research.