Hello, I believe the BSc is just an undergraduate degree whereas MSc is a masters (postgraduate level) course. The confusion may be stemming from the MSc you are looking at potentially being an integrated masters (meaning that it is an undergraduate and masters in one)? Overall, for psychology, the main thing you need to be aware of is if you are intending on going into the clinical field you will need to study a BPS accredited course. If you are unsure on which course may be the best fit for you, you can always email the admissions department for that university as they will be able to advise you!
-Kat (2nd year undergraduate psychology student at Lancaster University)